<?xml version="1.0" encoding="UTF-8"?>

<!--XBRL Document Created with the Workiva Platform-->
<!--Copyright 2024 Workiva-->
<!--r:8864d726-f81c-41f9-a3c8-ffb428e05070,g:d673dff3-7dd9-445f-baa9-0aaa66fce7ed-->
<link:linkbase xmlns:link="http://www.xbrl.org/2003/linkbase" xmlns:xlink="http://www.w3.org/1999/xlink" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:xbrldt="http://xbrl.org/2005/xbrldt" xsi:schemaLocation="http://www.xbrl.org/2003/linkbase http://www.xbrl.org/2003/xbrl-linkbase-2003-12-31.xsd">
  <link:arcroleRef arcroleURI="http://xbrl.org/int/dim/arcrole/all" xlink:type="simple" xlink:href="http://www.xbrl.org/2005/xbrldt-2005.xsd#all"/>
  <link:arcroleRef arcroleURI="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:type="simple" xlink:href="http://www.xbrl.org/2005/xbrldt-2005.xsd#hypercube-dimension"/>
  <link:arcroleRef arcroleURI="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:type="simple" xlink:href="http://www.xbrl.org/2005/xbrldt-2005.xsd#dimension-default"/>
  <link:arcroleRef arcroleURI="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:type="simple" xlink:href="http://www.xbrl.org/2005/xbrldt-2005.xsd#dimension-domain"/>
  <link:arcroleRef arcroleURI="http://xbrl.org/int/dim/arcrole/domain-member" xlink:type="simple" xlink:href="http://www.xbrl.org/2005/xbrldt-2005.xsd#domain-member"/>
  <link:roleRef roleURI="http://www.ngsgi.com/role/CONDENSEDCONSOLIDATEDSTATEMENTSOFOPERATIONS" xlink:type="simple" xlink:href="ngs-20240331.xsd#CONDENSEDCONSOLIDATEDSTATEMENTSOFOPERATIONS"/>
  <link:definitionLink xlink:role="http://www.ngsgi.com/role/CONDENSEDCONSOLIDATEDSTATEMENTSOFOPERATIONS" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementLineItems_f895bb8c-0336-437e-a293-87bb0235920f" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_StatementLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementTable_6c12c8e3-99b7-4fb2-b226-3966e4a968d7" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_StatementTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_StatementLineItems_f895bb8c-0336-437e-a293-87bb0235920f" xlink:to="loc_us-gaap_StatementTable_6c12c8e3-99b7-4fb2-b226-3966e4a968d7" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ProductOrServiceAxis_751c8b78-39da-41ef-bbb0-1457fc0081fc" xlink:href="https://xbrl.fasb.org/srt/2023/elts/srt-2023.xsd#srt_ProductOrServiceAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_StatementTable_6c12c8e3-99b7-4fb2-b226-3966e4a968d7" xlink:to="loc_srt_ProductOrServiceAxis_751c8b78-39da-41ef-bbb0-1457fc0081fc" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ProductsAndServicesDomain_751c8b78-39da-41ef-bbb0-1457fc0081fc_default" xlink:href="https://xbrl.fasb.org/srt/2023/elts/srt-2023.xsd#srt_ProductsAndServicesDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_ProductOrServiceAxis_751c8b78-39da-41ef-bbb0-1457fc0081fc" xlink:to="loc_srt_ProductsAndServicesDomain_751c8b78-39da-41ef-bbb0-1457fc0081fc_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ProductsAndServicesDomain_c567e163-5fe3-4c04-8e7e-d18f311a2c0d" xlink:href="https://xbrl.fasb.org/srt/2023/elts/srt-2023.xsd#srt_ProductsAndServicesDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_ProductOrServiceAxis_751c8b78-39da-41ef-bbb0-1457fc0081fc" xlink:to="loc_srt_ProductsAndServicesDomain_c567e163-5fe3-4c04-8e7e-d18f311a2c0d" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_ngs_ProductSalesMember_fb34b1f6-7556-43c1-8dc6-fd64db7ead5f" xlink:href="ngs-20240331.xsd#ngs_ProductSalesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_ProductsAndServicesDomain_c567e163-5fe3-4c04-8e7e-d18f311a2c0d" xlink:to="loc_ngs_ProductSalesMember_fb34b1f6-7556-43c1-8dc6-fd64db7ead5f" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_ngs_AftermarketServicesMember_7428fdb2-997f-4cc0-9d36-f6bb3305ab35" xlink:href="ngs-20240331.xsd#ngs_AftermarketServicesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_ProductsAndServicesDomain_c567e163-5fe3-4c04-8e7e-d18f311a2c0d" xlink:to="loc_ngs_AftermarketServicesMember_7428fdb2-997f-4cc0-9d36-f6bb3305ab35" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RevenuesAbstract_05d2aae8-f697-480f-a579-9977a7e5ba0f" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_RevenuesAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_f895bb8c-0336-437e-a293-87bb0235920f" xlink:to="loc_us-gaap_RevenuesAbstract_05d2aae8-f697-480f-a579-9977a7e5ba0f" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RentalIncomeNonoperating_b66af985-58ae-4609-a9d8-aa2817a46e67" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_RentalIncomeNonoperating"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RevenuesAbstract_05d2aae8-f697-480f-a579-9977a7e5ba0f" xlink:to="loc_us-gaap_RentalIncomeNonoperating_b66af985-58ae-4609-a9d8-aa2817a46e67" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RevenueFromContractWithCustomerExcludingAssessedTax_abd8280f-8343-474f-971c-7405c8eff47d" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_RevenueFromContractWithCustomerExcludingAssessedTax"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RevenuesAbstract_05d2aae8-f697-480f-a579-9977a7e5ba0f" xlink:to="loc_us-gaap_RevenueFromContractWithCustomerExcludingAssessedTax_abd8280f-8343-474f-971c-7405c8eff47d" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_Revenues_4436b3df-e509-4add-a2e6-73abfcc6c932" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_Revenues"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RevenuesAbstract_05d2aae8-f697-480f-a579-9977a7e5ba0f" xlink:to="loc_us-gaap_Revenues_4436b3df-e509-4add-a2e6-73abfcc6c932" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CostsAndExpensesAbstract_38353d7d-7af6-4b6c-adaf-1794ba5278f2" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_CostsAndExpensesAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_f895bb8c-0336-437e-a293-87bb0235920f" xlink:to="loc_us-gaap_CostsAndExpensesAbstract_38353d7d-7af6-4b6c-adaf-1794ba5278f2" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DirectCostsOfLeasedAndRentedPropertyOrEquipment_2849c86d-be5f-42ac-bd7d-f2a4e67bbbcb" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_DirectCostsOfLeasedAndRentedPropertyOrEquipment"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_CostsAndExpensesAbstract_38353d7d-7af6-4b6c-adaf-1794ba5278f2" xlink:to="loc_us-gaap_DirectCostsOfLeasedAndRentedPropertyOrEquipment_2849c86d-be5f-42ac-bd7d-f2a4e67bbbcb" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OtherCostOfOperatingRevenue_b726744f-0355-4892-b776-da563f9ffcc3" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_OtherCostOfOperatingRevenue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_CostsAndExpensesAbstract_38353d7d-7af6-4b6c-adaf-1794ba5278f2" xlink:to="loc_us-gaap_OtherCostOfOperatingRevenue_b726744f-0355-4892-b776-da563f9ffcc3" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CostOfPropertyRepairsAndMaintenance_ba9070ea-6509-4258-a1c9-8557cfee2094" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_CostOfPropertyRepairsAndMaintenance"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_CostsAndExpensesAbstract_38353d7d-7af6-4b6c-adaf-1794ba5278f2" xlink:to="loc_us-gaap_CostOfPropertyRepairsAndMaintenance_ba9070ea-6509-4258-a1c9-8557cfee2094" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SellingGeneralAndAdministrativeExpense_be48fbd6-5f7e-496b-ab03-d51dffaebfae" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_SellingGeneralAndAdministrativeExpense"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_CostsAndExpensesAbstract_38353d7d-7af6-4b6c-adaf-1794ba5278f2" xlink:to="loc_us-gaap_SellingGeneralAndAdministrativeExpense_be48fbd6-5f7e-496b-ab03-d51dffaebfae" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DepreciationDepletionAndAmortization_9d8ac1a3-0c90-42f9-b983-0ea9102cc737" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_DepreciationDepletionAndAmortization"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_CostsAndExpensesAbstract_38353d7d-7af6-4b6c-adaf-1794ba5278f2" xlink:to="loc_us-gaap_DepreciationDepletionAndAmortization_9d8ac1a3-0c90-42f9-b983-0ea9102cc737" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ImpairedAssetsToBeDisposedOfByMethodOtherThanSaleAmountOfImpairmentLoss_b058e34d-5e45-4832-af39-27188da63ff7" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_ImpairedAssetsToBeDisposedOfByMethodOtherThanSaleAmountOfImpairmentLoss"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_CostsAndExpensesAbstract_38353d7d-7af6-4b6c-adaf-1794ba5278f2" xlink:to="loc_us-gaap_ImpairedAssetsToBeDisposedOfByMethodOtherThanSaleAmountOfImpairmentLoss_b058e34d-5e45-4832-af39-27188da63ff7" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CostsAndExpenses_992c4f7c-ce7e-4262-9f47-07e21192bea9" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_CostsAndExpenses"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_CostsAndExpensesAbstract_38353d7d-7af6-4b6c-adaf-1794ba5278f2" xlink:to="loc_us-gaap_CostsAndExpenses_992c4f7c-ce7e-4262-9f47-07e21192bea9" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OperatingIncomeLoss_2f90e6e8-2c84-4364-9043-491bf0a2f90a" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_OperatingIncomeLoss"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_f895bb8c-0336-437e-a293-87bb0235920f" xlink:to="loc_us-gaap_OperatingIncomeLoss_2f90e6e8-2c84-4364-9043-491bf0a2f90a" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_NonoperatingIncomeExpenseAbstract_a3da7acf-bc67-435e-af7f-956beb2c29b7" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_NonoperatingIncomeExpenseAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_f895bb8c-0336-437e-a293-87bb0235920f" xlink:to="loc_us-gaap_NonoperatingIncomeExpenseAbstract_a3da7acf-bc67-435e-af7f-956beb2c29b7" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_InterestExpense_b45c6488-8482-4efe-89d7-34d62d3a1dbe" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_InterestExpense"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_NonoperatingIncomeExpenseAbstract_a3da7acf-bc67-435e-af7f-956beb2c29b7" xlink:to="loc_us-gaap_InterestExpense_b45c6488-8482-4efe-89d7-34d62d3a1dbe" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OtherNonoperatingIncomeExpense_179ed646-0d00-45f4-be11-85dbfa3321cb" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_OtherNonoperatingIncomeExpense"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_NonoperatingIncomeExpenseAbstract_a3da7acf-bc67-435e-af7f-956beb2c29b7" xlink:to="loc_us-gaap_OtherNonoperatingIncomeExpense_179ed646-0d00-45f4-be11-85dbfa3321cb" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_NonoperatingIncomeExpense_0fecd15f-2b6d-4615-9cd3-37a5ce965840" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_NonoperatingIncomeExpense"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_NonoperatingIncomeExpenseAbstract_a3da7acf-bc67-435e-af7f-956beb2c29b7" xlink:to="loc_us-gaap_NonoperatingIncomeExpense_0fecd15f-2b6d-4615-9cd3-37a5ce965840" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeLossFromContinuingOperationsBeforeIncomeTaxesExtraordinaryItemsNoncontrollingInterest_b9e1bd09-5772-439f-9d38-40184cbec6c6" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_IncomeLossFromContinuingOperationsBeforeIncomeTaxesExtraordinaryItemsNoncontrollingInterest"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_f895bb8c-0336-437e-a293-87bb0235920f" xlink:to="loc_us-gaap_IncomeLossFromContinuingOperationsBeforeIncomeTaxesExtraordinaryItemsNoncontrollingInterest_b9e1bd09-5772-439f-9d38-40184cbec6c6" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeTaxExpenseBenefit_1ce2c413-2098-4eca-a7af-b8590d8b93f6" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_IncomeTaxExpenseBenefit"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_f895bb8c-0336-437e-a293-87bb0235920f" xlink:to="loc_us-gaap_IncomeTaxExpenseBenefit_1ce2c413-2098-4eca-a7af-b8590d8b93f6" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_NetIncomeLoss_c834e418-7f36-4db8-9436-34380a3abc94" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_NetIncomeLoss"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_f895bb8c-0336-437e-a293-87bb0235920f" xlink:to="loc_us-gaap_NetIncomeLoss_c834e418-7f36-4db8-9436-34380a3abc94" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EarningsPerShareAbstract_d1310cc1-9da6-4926-812b-8521fdff4f83" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_EarningsPerShareAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_f895bb8c-0336-437e-a293-87bb0235920f" xlink:to="loc_us-gaap_EarningsPerShareAbstract_d1310cc1-9da6-4926-812b-8521fdff4f83" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EarningsPerShareBasic_3f036273-0a8e-4518-8279-22c6e169928a" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_EarningsPerShareBasic"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EarningsPerShareAbstract_d1310cc1-9da6-4926-812b-8521fdff4f83" xlink:to="loc_us-gaap_EarningsPerShareBasic_3f036273-0a8e-4518-8279-22c6e169928a" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EarningsPerShareDiluted_91b289d7-7ecd-461f-99fb-08ee0ccc4199" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_EarningsPerShareDiluted"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EarningsPerShareAbstract_d1310cc1-9da6-4926-812b-8521fdff4f83" xlink:to="loc_us-gaap_EarningsPerShareDiluted_91b289d7-7ecd-461f-99fb-08ee0ccc4199" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_ngs_WeightedAverageSharesOutstandingBasicAndDilutedAbstract_7dfbb7d2-5e50-4eaf-9cc1-ed59ab75c765" xlink:href="ngs-20240331.xsd#ngs_WeightedAverageSharesOutstandingBasicAndDilutedAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_f895bb8c-0336-437e-a293-87bb0235920f" xlink:to="loc_ngs_WeightedAverageSharesOutstandingBasicAndDilutedAbstract_7dfbb7d2-5e50-4eaf-9cc1-ed59ab75c765" xlink:type="arc" order="8"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_WeightedAverageNumberOfSharesOutstandingBasic_8d053dd2-011f-4045-8a04-ad097de91086" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_WeightedAverageNumberOfSharesOutstandingBasic"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_ngs_WeightedAverageSharesOutstandingBasicAndDilutedAbstract_7dfbb7d2-5e50-4eaf-9cc1-ed59ab75c765" xlink:to="loc_us-gaap_WeightedAverageNumberOfSharesOutstandingBasic_8d053dd2-011f-4045-8a04-ad097de91086" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_WeightedAverageNumberOfDilutedSharesOutstanding_d92cb0ef-2e48-4a3d-a9e9-8bc19ff42372" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_WeightedAverageNumberOfDilutedSharesOutstanding"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_ngs_WeightedAverageSharesOutstandingBasicAndDilutedAbstract_7dfbb7d2-5e50-4eaf-9cc1-ed59ab75c765" xlink:to="loc_us-gaap_WeightedAverageNumberOfDilutedSharesOutstanding_d92cb0ef-2e48-4a3d-a9e9-8bc19ff42372" xlink:type="arc" order="2"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.ngsgi.com/role/CONDENSEDCONSOLIDATEDSTATEMENTSOFSTOCKHOLDERSEQUITY" xlink:type="simple" xlink:href="ngs-20240331.xsd#CONDENSEDCONSOLIDATEDSTATEMENTSOFSTOCKHOLDERSEQUITY"/>
  <link:definitionLink xlink:role="http://www.ngsgi.com/role/CONDENSEDCONSOLIDATEDSTATEMENTSOFSTOCKHOLDERSEQUITY" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementLineItems_4644c6e9-2146-43c8-be25-57602acfe748" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_StatementLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementTable_c950f60e-9433-4c01-93f3-138d52775e1c" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_StatementTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_StatementLineItems_4644c6e9-2146-43c8-be25-57602acfe748" xlink:to="loc_us-gaap_StatementTable_c950f60e-9433-4c01-93f3-138d52775e1c" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementEquityComponentsAxis_04623fce-a2c9-4417-92cf-820ccb847218" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_StatementEquityComponentsAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_StatementTable_c950f60e-9433-4c01-93f3-138d52775e1c" xlink:to="loc_us-gaap_StatementEquityComponentsAxis_04623fce-a2c9-4417-92cf-820ccb847218" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EquityComponentDomain_04623fce-a2c9-4417-92cf-820ccb847218_default" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_EquityComponentDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_StatementEquityComponentsAxis_04623fce-a2c9-4417-92cf-820ccb847218" xlink:to="loc_us-gaap_EquityComponentDomain_04623fce-a2c9-4417-92cf-820ccb847218_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EquityComponentDomain_864ba975-faa4-4b57-a05f-3c17fcd4a060" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_EquityComponentDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_StatementEquityComponentsAxis_04623fce-a2c9-4417-92cf-820ccb847218" xlink:to="loc_us-gaap_EquityComponentDomain_864ba975-faa4-4b57-a05f-3c17fcd4a060" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PreferredStockMember_eeeaea83-397b-4885-8a3b-f34fa78ca59b" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_PreferredStockMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityComponentDomain_864ba975-faa4-4b57-a05f-3c17fcd4a060" xlink:to="loc_us-gaap_PreferredStockMember_eeeaea83-397b-4885-8a3b-f34fa78ca59b" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonStockMember_0f40b812-820d-493a-8e5f-54a7a73edc32" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_CommonStockMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityComponentDomain_864ba975-faa4-4b57-a05f-3c17fcd4a060" xlink:to="loc_us-gaap_CommonStockMember_0f40b812-820d-493a-8e5f-54a7a73edc32" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AdditionalPaidInCapitalMember_1f3dc392-0fa0-43f7-ab7c-0fb1cf40cb8a" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_AdditionalPaidInCapitalMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityComponentDomain_864ba975-faa4-4b57-a05f-3c17fcd4a060" xlink:to="loc_us-gaap_AdditionalPaidInCapitalMember_1f3dc392-0fa0-43f7-ab7c-0fb1cf40cb8a" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RetainedEarningsMember_8a43af29-a9ea-4bf6-806e-1bbf06afcdfb" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_RetainedEarningsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityComponentDomain_864ba975-faa4-4b57-a05f-3c17fcd4a060" xlink:to="loc_us-gaap_RetainedEarningsMember_8a43af29-a9ea-4bf6-806e-1bbf06afcdfb" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TreasuryStockCommonMember_cb4cdc81-82d6-431e-8a7c-ad6af340804f" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_TreasuryStockCommonMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityComponentDomain_864ba975-faa4-4b57-a05f-3c17fcd4a060" xlink:to="loc_us-gaap_TreasuryStockCommonMember_cb4cdc81-82d6-431e-8a7c-ad6af340804f" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_a7bdbb70-1198-43bf-bdb3-493e933d9120" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_IncreaseDecreaseInStockholdersEquityRollForward"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_4644c6e9-2146-43c8-be25-57602acfe748" xlink:to="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_a7bdbb70-1198-43bf-bdb3-493e933d9120" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PreferredStockSharesIssued_a29610a8-1f55-445a-8ba6-8ec4f871c741" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_PreferredStockSharesIssued"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_a7bdbb70-1198-43bf-bdb3-493e933d9120" xlink:to="loc_us-gaap_PreferredStockSharesIssued_a29610a8-1f55-445a-8ba6-8ec4f871c741" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockholdersEquity_0171911f-6b17-4406-aa73-ff991367cb99" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_StockholdersEquity"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_a7bdbb70-1198-43bf-bdb3-493e933d9120" xlink:to="loc_us-gaap_StockholdersEquity_0171911f-6b17-4406-aa73-ff991367cb99" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonStockSharesIssued_feba224d-e6f1-422d-90aa-b83bfa657c92" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_CommonStockSharesIssued"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_a7bdbb70-1198-43bf-bdb3-493e933d9120" xlink:to="loc_us-gaap_CommonStockSharesIssued_feba224d-e6f1-422d-90aa-b83bfa657c92" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TreasuryStockCommonShares_5923357f-57a2-46ee-b492-c71ae0ee5f02" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_TreasuryStockCommonShares"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_a7bdbb70-1198-43bf-bdb3-493e933d9120" xlink:to="loc_us-gaap_TreasuryStockCommonShares_5923357f-57a2-46ee-b492-c71ae0ee5f02" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AdjustmentsToAdditionalPaidInCapitalSharebasedCompensationRequisiteServicePeriodRecognitionValue_d0d7f18f-f039-43e8-bd9d-c8479425baf9" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_AdjustmentsToAdditionalPaidInCapitalSharebasedCompensationRequisiteServicePeriodRecognitionValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_a7bdbb70-1198-43bf-bdb3-493e933d9120" xlink:to="loc_us-gaap_AdjustmentsToAdditionalPaidInCapitalSharebasedCompensationRequisiteServicePeriodRecognitionValue_d0d7f18f-f039-43e8-bd9d-c8479425baf9" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockIssuedDuringPeriodSharesRestrictedStockAwardNetOfForfeitures_2b24415e-64ca-48f8-83aa-35c8977ab4e1" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_StockIssuedDuringPeriodSharesRestrictedStockAwardNetOfForfeitures"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_a7bdbb70-1198-43bf-bdb3-493e933d9120" xlink:to="loc_us-gaap_StockIssuedDuringPeriodSharesRestrictedStockAwardNetOfForfeitures_2b24415e-64ca-48f8-83aa-35c8977ab4e1" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AdjustmentsToAdditionalPaidInCapitalShareBasedCompensationRestrictedStockUnitsRequisiteServicePeriodRecognition_f89033cf-aaea-484c-a28d-fc77eb5c94ef" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_AdjustmentsToAdditionalPaidInCapitalShareBasedCompensationRestrictedStockUnitsRequisiteServicePeriodRecognition"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_a7bdbb70-1198-43bf-bdb3-493e933d9120" xlink:to="loc_us-gaap_AdjustmentsToAdditionalPaidInCapitalShareBasedCompensationRestrictedStockUnitsRequisiteServicePeriodRecognition_f89033cf-aaea-484c-a28d-fc77eb5c94ef" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SharesPaidForTaxWithholdingForShareBasedCompensation_f96f4e33-974a-4b97-88d2-095ac772bbc3" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_SharesPaidForTaxWithholdingForShareBasedCompensation"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_a7bdbb70-1198-43bf-bdb3-493e933d9120" xlink:to="loc_us-gaap_SharesPaidForTaxWithholdingForShareBasedCompensation_f96f4e33-974a-4b97-88d2-095ac772bbc3" xlink:type="arc" order="8"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AdjustmentsRelatedToTaxWithholdingForShareBasedCompensation_f8db3ee6-09cb-46a0-a4d7-4b41e274cef1" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_AdjustmentsRelatedToTaxWithholdingForShareBasedCompensation"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_a7bdbb70-1198-43bf-bdb3-493e933d9120" xlink:to="loc_us-gaap_AdjustmentsRelatedToTaxWithholdingForShareBasedCompensation_f8db3ee6-09cb-46a0-a4d7-4b41e274cef1" xlink:type="arc" order="9"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_NetIncomeLoss_ee9c329f-7ac5-4bea-b5c0-a57a6ded3b5b" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_NetIncomeLoss"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_a7bdbb70-1198-43bf-bdb3-493e933d9120" xlink:to="loc_us-gaap_NetIncomeLoss_ee9c329f-7ac5-4bea-b5c0-a57a6ded3b5b" xlink:type="arc" order="10"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PreferredStockSharesIssued_e300a079-674e-4faf-80aa-eeadf1dc7235" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_PreferredStockSharesIssued"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockholdersEquity_5b79fd95-f266-4faf-88df-1de001c5b9ce" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_StockholdersEquity"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonStockSharesIssued_0e013eaa-bd35-4395-bebf-902c98bf2325" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_CommonStockSharesIssued"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TreasuryStockCommonShares_228ae840-8709-4ad2-932c-d0cf376032a1" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_TreasuryStockCommonShares"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.ngsgi.com/role/SummaryofSignificantAccountingPoliciesNarrativeDetails" xlink:type="simple" xlink:href="ngs-20240331.xsd#SummaryofSignificantAccountingPoliciesNarrativeDetails"/>
  <link:definitionLink xlink:role="http://www.ngsgi.com/role/SummaryofSignificantAccountingPoliciesNarrativeDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EntityWideInformationRevenueFromExternalCustomerLineItems_d9b325f8-b254-4cbb-ab38-782cd302ce05" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_EntityWideInformationRevenueFromExternalCustomerLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfEntityWideInformationRevenueFromExternalCustomersByProductsAndServicesTable_986c50fe-7405-4464-89b7-9a04e6b6de55" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_ScheduleOfEntityWideInformationRevenueFromExternalCustomersByProductsAndServicesTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_EntityWideInformationRevenueFromExternalCustomerLineItems_d9b325f8-b254-4cbb-ab38-782cd302ce05" xlink:to="loc_us-gaap_ScheduleOfEntityWideInformationRevenueFromExternalCustomersByProductsAndServicesTable_986c50fe-7405-4464-89b7-9a04e6b6de55" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LeaseContractualTermAxis_33bb6170-1c57-4429-802e-c5f7dda4d27b" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_LeaseContractualTermAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfEntityWideInformationRevenueFromExternalCustomersByProductsAndServicesTable_986c50fe-7405-4464-89b7-9a04e6b6de55" xlink:to="loc_us-gaap_LeaseContractualTermAxis_33bb6170-1c57-4429-802e-c5f7dda4d27b" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LeaseContractualTermDomain_33bb6170-1c57-4429-802e-c5f7dda4d27b_default" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_LeaseContractualTermDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_LeaseContractualTermAxis_33bb6170-1c57-4429-802e-c5f7dda4d27b" xlink:to="loc_us-gaap_LeaseContractualTermDomain_33bb6170-1c57-4429-802e-c5f7dda4d27b_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LeaseContractualTermDomain_04705486-fe8d-44fb-997a-71b02d1aa13e" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_LeaseContractualTermDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_LeaseContractualTermAxis_33bb6170-1c57-4429-802e-c5f7dda4d27b" xlink:to="loc_us-gaap_LeaseContractualTermDomain_04705486-fe8d-44fb-997a-71b02d1aa13e" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_ngs_RentalContractsExcludingLargeHorsepowerCompressorsMember_e86f1910-c577-45de-87a5-3144f28aee94" xlink:href="ngs-20240331.xsd#ngs_RentalContractsExcludingLargeHorsepowerCompressorsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LeaseContractualTermDomain_04705486-fe8d-44fb-997a-71b02d1aa13e" xlink:to="loc_ngs_RentalContractsExcludingLargeHorsepowerCompressorsMember_e86f1910-c577-45de-87a5-3144f28aee94" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_ngs_RentalContractsLargeHorsepowerCompressorsMember_dd0b29ca-2ebc-4234-9a16-195dc5d2c9cb" xlink:href="ngs-20240331.xsd#ngs_RentalContractsLargeHorsepowerCompressorsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LeaseContractualTermDomain_04705486-fe8d-44fb-997a-71b02d1aa13e" xlink:to="loc_ngs_RentalContractsLargeHorsepowerCompressorsMember_dd0b29ca-2ebc-4234-9a16-195dc5d2c9cb" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeAxis_f71815dc-edbf-462e-a2e9-8b7aa158c264" xlink:href="https://xbrl.fasb.org/srt/2023/elts/srt-2023.xsd#srt_RangeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfEntityWideInformationRevenueFromExternalCustomersByProductsAndServicesTable_986c50fe-7405-4464-89b7-9a04e6b6de55" xlink:to="loc_srt_RangeAxis_f71815dc-edbf-462e-a2e9-8b7aa158c264" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeMember_f71815dc-edbf-462e-a2e9-8b7aa158c264_default" xlink:href="https://xbrl.fasb.org/srt/2023/elts/srt-2023.xsd#srt_RangeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_RangeAxis_f71815dc-edbf-462e-a2e9-8b7aa158c264" xlink:to="loc_srt_RangeMember_f71815dc-edbf-462e-a2e9-8b7aa158c264_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeMember_851923fe-9494-4f27-ae59-9388595aaea8" xlink:href="https://xbrl.fasb.org/srt/2023/elts/srt-2023.xsd#srt_RangeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_RangeAxis_f71815dc-edbf-462e-a2e9-8b7aa158c264" xlink:to="loc_srt_RangeMember_851923fe-9494-4f27-ae59-9388595aaea8" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_MinimumMember_9ac81b8f-ae68-42c0-8477-b2993b778fab" xlink:href="https://xbrl.fasb.org/srt/2023/elts/srt-2023.xsd#srt_MinimumMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RangeMember_851923fe-9494-4f27-ae59-9388595aaea8" xlink:to="loc_srt_MinimumMember_9ac81b8f-ae68-42c0-8477-b2993b778fab" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_MaximumMember_4322f0ad-c67a-46fc-bb85-6b0af323e777" xlink:href="https://xbrl.fasb.org/srt/2023/elts/srt-2023.xsd#srt_MaximumMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RangeMember_851923fe-9494-4f27-ae59-9388595aaea8" xlink:to="loc_srt_MaximumMember_4322f0ad-c67a-46fc-bb85-6b0af323e777" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LessorOperatingLeaseTermOfContract_7f09ea1a-317a-4e50-9d87-02d692c46830" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_LessorOperatingLeaseTermOfContract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EntityWideInformationRevenueFromExternalCustomerLineItems_d9b325f8-b254-4cbb-ab38-782cd302ce05" xlink:to="loc_us-gaap_LessorOperatingLeaseTermOfContract_7f09ea1a-317a-4e50-9d87-02d692c46830" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_Revenues_147f24bb-f3cf-4b97-8402-08bd5e0d28f5" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_Revenues"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EntityWideInformationRevenueFromExternalCustomerLineItems_d9b325f8-b254-4cbb-ab38-782cd302ce05" xlink:to="loc_us-gaap_Revenues_147f24bb-f3cf-4b97-8402-08bd5e0d28f5" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_ngs_ContractWithCustomerRevenueRecognizedIncludingOpeningBalance_d97af47a-73e2-432c-ab0a-b6e454f8206e" xlink:href="ngs-20240331.xsd#ngs_ContractWithCustomerRevenueRecognizedIncludingOpeningBalance"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EntityWideInformationRevenueFromExternalCustomerLineItems_d9b325f8-b254-4cbb-ab38-782cd302ce05" xlink:to="loc_ngs_ContractWithCustomerRevenueRecognizedIncludingOpeningBalance_d97af47a-73e2-432c-ab0a-b6e454f8206e" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ContractWithCustomerLiabilityCurrent_2a447e8d-257a-4ad4-bb88-b0ed88615de1" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_ContractWithCustomerLiabilityCurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EntityWideInformationRevenueFromExternalCustomerLineItems_d9b325f8-b254-4cbb-ab38-782cd302ce05" xlink:to="loc_us-gaap_ContractWithCustomerLiabilityCurrent_2a447e8d-257a-4ad4-bb88-b0ed88615de1" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CapitalizedContractCostAmortizationPeriod_c3cc6464-33d4-44b1-86fa-ab8c8bed6007" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_CapitalizedContractCostAmortizationPeriod"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EntityWideInformationRevenueFromExternalCustomerLineItems_d9b325f8-b254-4cbb-ab38-782cd302ce05" xlink:to="loc_us-gaap_CapitalizedContractCostAmortizationPeriod_c3cc6464-33d4-44b1-86fa-ab8c8bed6007" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LiabilityForUncertainTaxPositionsCurrent_0d2a3589-05be-46c9-af87-14f5d944e62e" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_LiabilityForUncertainTaxPositionsCurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EntityWideInformationRevenueFromExternalCustomerLineItems_d9b325f8-b254-4cbb-ab38-782cd302ce05" xlink:to="loc_us-gaap_LiabilityForUncertainTaxPositionsCurrent_0d2a3589-05be-46c9-af87-14f5d944e62e" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_InterestCostsCapitalized_e49e1745-ffd5-4c48-b15a-f2ce361a0ebd" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_InterestCostsCapitalized"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EntityWideInformationRevenueFromExternalCustomerLineItems_d9b325f8-b254-4cbb-ab38-782cd302ce05" xlink:to="loc_us-gaap_InterestCostsCapitalized_e49e1745-ffd5-4c48-b15a-f2ce361a0ebd" xlink:type="arc" order="6"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.ngsgi.com/role/SummaryofSignificantAccountingPoliciesScheduleofDisaggregationofRevenueDetails" xlink:type="simple" xlink:href="ngs-20240331.xsd#SummaryofSignificantAccountingPoliciesScheduleofDisaggregationofRevenueDetails"/>
  <link:definitionLink xlink:role="http://www.ngsgi.com/role/SummaryofSignificantAccountingPoliciesScheduleofDisaggregationofRevenueDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DisaggregationOfRevenueLineItems_d10d3898-4a04-45d9-9957-935afb39bd8c" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_DisaggregationOfRevenueLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DisaggregationOfRevenueTable_62a8709c-d374-46bd-97b5-3537912f2ac9" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_DisaggregationOfRevenueTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_DisaggregationOfRevenueLineItems_d10d3898-4a04-45d9-9957-935afb39bd8c" xlink:to="loc_us-gaap_DisaggregationOfRevenueTable_62a8709c-d374-46bd-97b5-3537912f2ac9" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ProductOrServiceAxis_3a006079-9453-4135-873a-9156f48d76c0" xlink:href="https://xbrl.fasb.org/srt/2023/elts/srt-2023.xsd#srt_ProductOrServiceAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DisaggregationOfRevenueTable_62a8709c-d374-46bd-97b5-3537912f2ac9" xlink:to="loc_srt_ProductOrServiceAxis_3a006079-9453-4135-873a-9156f48d76c0" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ProductsAndServicesDomain_3a006079-9453-4135-873a-9156f48d76c0_default" xlink:href="https://xbrl.fasb.org/srt/2023/elts/srt-2023.xsd#srt_ProductsAndServicesDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_ProductOrServiceAxis_3a006079-9453-4135-873a-9156f48d76c0" xlink:to="loc_srt_ProductsAndServicesDomain_3a006079-9453-4135-873a-9156f48d76c0_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ProductsAndServicesDomain_8ac7c27c-f596-4706-95f7-c1073f986d77" xlink:href="https://xbrl.fasb.org/srt/2023/elts/srt-2023.xsd#srt_ProductsAndServicesDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_ProductOrServiceAxis_3a006079-9453-4135-873a-9156f48d76c0" xlink:to="loc_srt_ProductsAndServicesDomain_8ac7c27c-f596-4706-95f7-c1073f986d77" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_ngs_CompressorRelatedMember_1e74dba2-8090-49d2-a53b-eb987b5ecdf9" xlink:href="ngs-20240331.xsd#ngs_CompressorRelatedMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_ProductsAndServicesDomain_8ac7c27c-f596-4706-95f7-c1073f986d77" xlink:to="loc_ngs_CompressorRelatedMember_1e74dba2-8090-49d2-a53b-eb987b5ecdf9" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_ngs_OtherPartsRebuildsMember_76e9be0b-8395-45a1-8052-77540520b855" xlink:href="ngs-20240331.xsd#ngs_OtherPartsRebuildsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_ProductsAndServicesDomain_8ac7c27c-f596-4706-95f7-c1073f986d77" xlink:to="loc_ngs_OtherPartsRebuildsMember_76e9be0b-8395-45a1-8052-77540520b855" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_ngs_AftermarketServicesMember_d78ad802-0128-409d-b8eb-6d3ecaee88e2" xlink:href="ngs-20240331.xsd#ngs_AftermarketServicesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_ProductsAndServicesDomain_8ac7c27c-f596-4706-95f7-c1073f986d77" xlink:to="loc_ngs_AftermarketServicesMember_d78ad802-0128-409d-b8eb-6d3ecaee88e2" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RevenueFromContractWithCustomerExcludingAssessedTax_fed7beeb-7de2-4ad0-94b9-14fe9cdbe513" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_RevenueFromContractWithCustomerExcludingAssessedTax"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DisaggregationOfRevenueLineItems_d10d3898-4a04-45d9-9957-935afb39bd8c" xlink:to="loc_us-gaap_RevenueFromContractWithCustomerExcludingAssessedTax_fed7beeb-7de2-4ad0-94b9-14fe9cdbe513" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RentalIncomeNonoperating_027fbf08-1236-4169-87f8-136360dfe893" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_RentalIncomeNonoperating"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DisaggregationOfRevenueLineItems_d10d3898-4a04-45d9-9957-935afb39bd8c" xlink:to="loc_us-gaap_RentalIncomeNonoperating_027fbf08-1236-4169-87f8-136360dfe893" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_Revenues_e3ea55b0-3b91-4e39-9cbf-5062491c667b" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_Revenues"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DisaggregationOfRevenueLineItems_d10d3898-4a04-45d9-9957-935afb39bd8c" xlink:to="loc_us-gaap_Revenues_e3ea55b0-3b91-4e39-9cbf-5062491c667b" xlink:type="arc" order="2"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.ngsgi.com/role/RentalEquipmentDetails" xlink:type="simple" xlink:href="ngs-20240331.xsd#RentalEquipmentDetails"/>
  <link:definitionLink xlink:role="http://www.ngsgi.com/role/RentalEquipmentDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PropertyPlantAndEquipmentLineItems_60014527-d01e-4f56-ab45-1d8e9f94a963" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_PropertyPlantAndEquipmentLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfPropertyPlantAndEquipmentTable_701f6455-0ba7-4695-9e7d-576df3f05d4e" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_ScheduleOfPropertyPlantAndEquipmentTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentLineItems_60014527-d01e-4f56-ab45-1d8e9f94a963" xlink:to="loc_us-gaap_ScheduleOfPropertyPlantAndEquipmentTable_701f6455-0ba7-4695-9e7d-576df3f05d4e" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PropertyPlantAndEquipmentByTypeAxis_2831bcaf-86f6-453c-be9a-2eef6dd1ab08" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_PropertyPlantAndEquipmentByTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfPropertyPlantAndEquipmentTable_701f6455-0ba7-4695-9e7d-576df3f05d4e" xlink:to="loc_us-gaap_PropertyPlantAndEquipmentByTypeAxis_2831bcaf-86f6-453c-be9a-2eef6dd1ab08" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_2831bcaf-86f6-453c-be9a-2eef6dd1ab08_default" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_PropertyPlantAndEquipmentTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentByTypeAxis_2831bcaf-86f6-453c-be9a-2eef6dd1ab08" xlink:to="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_2831bcaf-86f6-453c-be9a-2eef6dd1ab08_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_43723c2e-b0cb-4af1-9464-7a2c0cf8fabb" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_PropertyPlantAndEquipmentTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentByTypeAxis_2831bcaf-86f6-453c-be9a-2eef6dd1ab08" xlink:to="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_43723c2e-b0cb-4af1-9464-7a2c0cf8fabb" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_ngs_RentalCompressorUnitMember_80f67395-9943-4aa5-8a23-097722160466" xlink:href="ngs-20240331.xsd#ngs_RentalCompressorUnitMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_43723c2e-b0cb-4af1-9464-7a2c0cf8fabb" xlink:to="loc_ngs_RentalCompressorUnitMember_80f67395-9943-4aa5-8a23-097722160466" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_ngs_RentalCompressorUnitWorkInProgressMember_cbf979af-ef4c-404c-89b6-b933371701aa" xlink:href="ngs-20240331.xsd#ngs_RentalCompressorUnitWorkInProgressMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_43723c2e-b0cb-4af1-9464-7a2c0cf8fabb" xlink:to="loc_ngs_RentalCompressorUnitWorkInProgressMember_cbf979af-ef4c-404c-89b6-b933371701aa" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_ngs_RentalEquipmentGross_e1f82ad1-1112-4b89-81bd-4f3dd9035863" xlink:href="ngs-20240331.xsd#ngs_RentalEquipmentGross"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentLineItems_60014527-d01e-4f56-ab45-1d8e9f94a963" xlink:to="loc_ngs_RentalEquipmentGross_e1f82ad1-1112-4b89-81bd-4f3dd9035863" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_ngs_RentalEquipmentAccumulatedDepreciation_abf45373-84f2-4787-bef8-94aa08e3dff7" xlink:href="ngs-20240331.xsd#ngs_RentalEquipmentAccumulatedDepreciation"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentLineItems_60014527-d01e-4f56-ab45-1d8e9f94a963" xlink:to="loc_ngs_RentalEquipmentAccumulatedDepreciation_abf45373-84f2-4787-bef8-94aa08e3dff7" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_ngs_RentalEquipmentNet_bc720458-c5c0-469c-a25d-50e17d57b78f" xlink:href="ngs-20240331.xsd#ngs_RentalEquipmentNet"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentLineItems_60014527-d01e-4f56-ab45-1d8e9f94a963" xlink:to="loc_ngs_RentalEquipmentNet_bc720458-c5c0-469c-a25d-50e17d57b78f" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ImpairmentOfLongLivedAssetsHeldForUse_0eb909ad-d0a9-44a9-a80c-0b3ae456b0a7" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_ImpairmentOfLongLivedAssetsHeldForUse"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentLineItems_60014527-d01e-4f56-ab45-1d8e9f94a963" xlink:to="loc_us-gaap_ImpairmentOfLongLivedAssetsHeldForUse_0eb909ad-d0a9-44a9-a80c-0b3ae456b0a7" xlink:type="arc" order="3"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.ngsgi.com/role/LongtermDebtNarrativeDetails" xlink:type="simple" xlink:href="ngs-20240331.xsd#LongtermDebtNarrativeDetails"/>
  <link:definitionLink xlink:role="http://www.ngsgi.com/role/LongtermDebtNarrativeDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LineOfCreditFacilityLineItems_cd7ded0d-76bd-44ad-aa0a-2507b963167d" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_LineOfCreditFacilityLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LineOfCreditFacilityTable_4ba6b926-9e99-4a5a-b31f-790652369fb8" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_LineOfCreditFacilityTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_LineOfCreditFacilityLineItems_cd7ded0d-76bd-44ad-aa0a-2507b963167d" xlink:to="loc_us-gaap_LineOfCreditFacilityTable_4ba6b926-9e99-4a5a-b31f-790652369fb8" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CreditFacilityAxis_a4045099-f703-4d5d-8a2d-1306c20f5b5e" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_CreditFacilityAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_LineOfCreditFacilityTable_4ba6b926-9e99-4a5a-b31f-790652369fb8" xlink:to="loc_us-gaap_CreditFacilityAxis_a4045099-f703-4d5d-8a2d-1306c20f5b5e" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CreditFacilityDomain_a4045099-f703-4d5d-8a2d-1306c20f5b5e_default" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_CreditFacilityDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_CreditFacilityAxis_a4045099-f703-4d5d-8a2d-1306c20f5b5e" xlink:to="loc_us-gaap_CreditFacilityDomain_a4045099-f703-4d5d-8a2d-1306c20f5b5e_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CreditFacilityDomain_bad892ae-faf7-42d9-a4d6-137f531899cf" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_CreditFacilityDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_CreditFacilityAxis_a4045099-f703-4d5d-8a2d-1306c20f5b5e" xlink:to="loc_us-gaap_CreditFacilityDomain_bad892ae-faf7-42d9-a4d6-137f531899cf" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RevolvingCreditFacilityMember_8e04efe8-b7e6-4b50-8b94-ae09842e845a" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_RevolvingCreditFacilityMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_CreditFacilityDomain_bad892ae-faf7-42d9-a4d6-137f531899cf" xlink:to="loc_us-gaap_RevolvingCreditFacilityMember_8e04efe8-b7e6-4b50-8b94-ae09842e845a" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentAxis_4aeb4a9f-bc45-4727-8466-6b9dead57ad8" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_DebtInstrumentAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_LineOfCreditFacilityTable_4ba6b926-9e99-4a5a-b31f-790652369fb8" xlink:to="loc_us-gaap_DebtInstrumentAxis_4aeb4a9f-bc45-4727-8466-6b9dead57ad8" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentNameDomain_4aeb4a9f-bc45-4727-8466-6b9dead57ad8_default" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_DebtInstrumentNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_DebtInstrumentAxis_4aeb4a9f-bc45-4727-8466-6b9dead57ad8" xlink:to="loc_us-gaap_DebtInstrumentNameDomain_4aeb4a9f-bc45-4727-8466-6b9dead57ad8_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentNameDomain_76eb94ee-01ab-498f-8572-1432452a8318" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_DebtInstrumentNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_DebtInstrumentAxis_4aeb4a9f-bc45-4727-8466-6b9dead57ad8" xlink:to="loc_us-gaap_DebtInstrumentNameDomain_76eb94ee-01ab-498f-8572-1432452a8318" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_ngs_NewCreditAgreementMember_a504b306-3b26-4cc4-afb2-153ed5cdb821" xlink:href="ngs-20240331.xsd#ngs_NewCreditAgreementMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_76eb94ee-01ab-498f-8572-1432452a8318" xlink:to="loc_ngs_NewCreditAgreementMember_a504b306-3b26-4cc4-afb2-153ed5cdb821" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongtermDebtTypeAxis_1c40a962-f7eb-492a-bb86-737760a49035" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_LongtermDebtTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_LineOfCreditFacilityTable_4ba6b926-9e99-4a5a-b31f-790652369fb8" xlink:to="loc_us-gaap_LongtermDebtTypeAxis_1c40a962-f7eb-492a-bb86-737760a49035" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongtermDebtTypeDomain_1c40a962-f7eb-492a-bb86-737760a49035_default" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_LongtermDebtTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_LongtermDebtTypeAxis_1c40a962-f7eb-492a-bb86-737760a49035" xlink:to="loc_us-gaap_LongtermDebtTypeDomain_1c40a962-f7eb-492a-bb86-737760a49035_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongtermDebtTypeDomain_0de1a0b4-b974-4e3f-8282-7e13c3d29c8e" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_LongtermDebtTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_LongtermDebtTypeAxis_1c40a962-f7eb-492a-bb86-737760a49035" xlink:to="loc_us-gaap_LongtermDebtTypeDomain_0de1a0b4-b974-4e3f-8282-7e13c3d29c8e" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LineOfCreditMember_dcce67ab-6935-4098-8baa-0b1651b3cb7d" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_LineOfCreditMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LongtermDebtTypeDomain_0de1a0b4-b974-4e3f-8282-7e13c3d29c8e" xlink:to="loc_us-gaap_LineOfCreditMember_dcce67ab-6935-4098-8baa-0b1651b3cb7d" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_VariableRateAxis_f7a05b78-7c5c-4902-aa3f-eb9c83406aa9" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_VariableRateAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_LineOfCreditFacilityTable_4ba6b926-9e99-4a5a-b31f-790652369fb8" xlink:to="loc_us-gaap_VariableRateAxis_f7a05b78-7c5c-4902-aa3f-eb9c83406aa9" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_VariableRateDomain_f7a05b78-7c5c-4902-aa3f-eb9c83406aa9_default" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_VariableRateDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_VariableRateAxis_f7a05b78-7c5c-4902-aa3f-eb9c83406aa9" xlink:to="loc_us-gaap_VariableRateDomain_f7a05b78-7c5c-4902-aa3f-eb9c83406aa9_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_VariableRateDomain_f0ac386b-685e-4443-b608-badec5bc2a2e" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_VariableRateDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_VariableRateAxis_f7a05b78-7c5c-4902-aa3f-eb9c83406aa9" xlink:to="loc_us-gaap_VariableRateDomain_f0ac386b-685e-4443-b608-badec5bc2a2e" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_ngs_FederalFundRateMember_061768e4-8fd7-4b92-b593-128410d9765f" xlink:href="ngs-20240331.xsd#ngs_FederalFundRateMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_VariableRateDomain_f0ac386b-685e-4443-b608-badec5bc2a2e" xlink:to="loc_ngs_FederalFundRateMember_061768e4-8fd7-4b92-b593-128410d9765f" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SecuredOvernightFinancingRateSofrOvernightIndexSwapRateMember_ef0193fa-c4cf-4ebf-827e-9ba4b1af478a" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_SecuredOvernightFinancingRateSofrOvernightIndexSwapRateMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_VariableRateDomain_f0ac386b-685e-4443-b608-badec5bc2a2e" xlink:to="loc_us-gaap_SecuredOvernightFinancingRateSofrOvernightIndexSwapRateMember_ef0193fa-c4cf-4ebf-827e-9ba4b1af478a" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_ngs_BaseRateLoansMember_b2897b19-ad1a-44e5-80d7-6ef445106b58" xlink:href="ngs-20240331.xsd#ngs_BaseRateLoansMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_VariableRateDomain_f0ac386b-685e-4443-b608-badec5bc2a2e" xlink:to="loc_ngs_BaseRateLoansMember_b2897b19-ad1a-44e5-80d7-6ef445106b58" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeAxis_dd486514-4f81-42fb-9116-9186c1a94ac6" xlink:href="https://xbrl.fasb.org/srt/2023/elts/srt-2023.xsd#srt_RangeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_LineOfCreditFacilityTable_4ba6b926-9e99-4a5a-b31f-790652369fb8" xlink:to="loc_srt_RangeAxis_dd486514-4f81-42fb-9116-9186c1a94ac6" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeMember_dd486514-4f81-42fb-9116-9186c1a94ac6_default" xlink:href="https://xbrl.fasb.org/srt/2023/elts/srt-2023.xsd#srt_RangeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_RangeAxis_dd486514-4f81-42fb-9116-9186c1a94ac6" xlink:to="loc_srt_RangeMember_dd486514-4f81-42fb-9116-9186c1a94ac6_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeMember_22efae9b-c6dd-46e3-a77c-aab65b49f566" xlink:href="https://xbrl.fasb.org/srt/2023/elts/srt-2023.xsd#srt_RangeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_RangeAxis_dd486514-4f81-42fb-9116-9186c1a94ac6" xlink:to="loc_srt_RangeMember_22efae9b-c6dd-46e3-a77c-aab65b49f566" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_MinimumMember_0d15ea7a-3838-4b17-b9d6-dcaeb5d2ad4a" xlink:href="https://xbrl.fasb.org/srt/2023/elts/srt-2023.xsd#srt_MinimumMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RangeMember_22efae9b-c6dd-46e3-a77c-aab65b49f566" xlink:to="loc_srt_MinimumMember_0d15ea7a-3838-4b17-b9d6-dcaeb5d2ad4a" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_MaximumMember_6cc79bbd-5b0b-4fe7-ac6d-cdff7227ddb2" xlink:href="https://xbrl.fasb.org/srt/2023/elts/srt-2023.xsd#srt_MaximumMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RangeMember_22efae9b-c6dd-46e3-a77c-aab65b49f566" xlink:to="loc_srt_MaximumMember_6cc79bbd-5b0b-4fe7-ac6d-cdff7227ddb2" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_StatementScenarioAxis_f42ac2da-ce21-483c-8d2b-420b358a02c6" xlink:href="https://xbrl.fasb.org/srt/2023/elts/srt-2023.xsd#srt_StatementScenarioAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_LineOfCreditFacilityTable_4ba6b926-9e99-4a5a-b31f-790652369fb8" xlink:to="loc_srt_StatementScenarioAxis_f42ac2da-ce21-483c-8d2b-420b358a02c6" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ScenarioUnspecifiedDomain_f42ac2da-ce21-483c-8d2b-420b358a02c6_default" xlink:href="https://xbrl.fasb.org/srt/2023/elts/srt-2023.xsd#srt_ScenarioUnspecifiedDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_StatementScenarioAxis_f42ac2da-ce21-483c-8d2b-420b358a02c6" xlink:to="loc_srt_ScenarioUnspecifiedDomain_f42ac2da-ce21-483c-8d2b-420b358a02c6_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ScenarioUnspecifiedDomain_3390e2a5-e930-4737-ba27-83630ffb1005" xlink:href="https://xbrl.fasb.org/srt/2023/elts/srt-2023.xsd#srt_ScenarioUnspecifiedDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_StatementScenarioAxis_f42ac2da-ce21-483c-8d2b-420b358a02c6" xlink:to="loc_srt_ScenarioUnspecifiedDomain_3390e2a5-e930-4737-ba27-83630ffb1005" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ScenarioForecastMember_752c3d07-3c5f-4e4b-a16e-df53e4a431cd" xlink:href="https://xbrl.fasb.org/srt/2023/elts/srt-2023.xsd#srt_ScenarioForecastMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_ScenarioUnspecifiedDomain_3390e2a5-e930-4737-ba27-83630ffb1005" xlink:to="loc_srt_ScenarioForecastMember_752c3d07-3c5f-4e4b-a16e-df53e4a431cd" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentTerm_5fbf0cdb-5bc3-4d4b-994e-238c366e5697" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_DebtInstrumentTerm"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LineOfCreditFacilityLineItems_cd7ded0d-76bd-44ad-aa0a-2507b963167d" xlink:to="loc_us-gaap_DebtInstrumentTerm_5fbf0cdb-5bc3-4d4b-994e-238c366e5697" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LineOfCreditFacilityCurrentBorrowingCapacity_efa256b1-81c0-4a25-ae10-11e9d8dae997" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_LineOfCreditFacilityCurrentBorrowingCapacity"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LineOfCreditFacilityLineItems_cd7ded0d-76bd-44ad-aa0a-2507b963167d" xlink:to="loc_us-gaap_LineOfCreditFacilityCurrentBorrowingCapacity_efa256b1-81c0-4a25-ae10-11e9d8dae997" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LineOfCreditFacilityMaximumBorrowingCapacity_1288f9b4-b027-455c-9a4b-4c86be76afb0" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_LineOfCreditFacilityMaximumBorrowingCapacity"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LineOfCreditFacilityLineItems_cd7ded0d-76bd-44ad-aa0a-2507b963167d" xlink:to="loc_us-gaap_LineOfCreditFacilityMaximumBorrowingCapacity_1288f9b4-b027-455c-9a4b-4c86be76afb0" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_ngs_LineOfCreditFacilityPotentialIncreaseInBorrowingCapacity_b49b52f5-b3fc-440c-a8ab-c476b34c3c91" xlink:href="ngs-20240331.xsd#ngs_LineOfCreditFacilityPotentialIncreaseInBorrowingCapacity"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LineOfCreditFacilityLineItems_cd7ded0d-76bd-44ad-aa0a-2507b963167d" xlink:to="loc_ngs_LineOfCreditFacilityPotentialIncreaseInBorrowingCapacity_b49b52f5-b3fc-440c-a8ab-c476b34c3c91" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_ngs_LineOfCreditFacilityPotentialMaximumBorrowingCapacity_0496d7bc-208d-4b04-ae27-cb0cc5420885" xlink:href="ngs-20240331.xsd#ngs_LineOfCreditFacilityPotentialMaximumBorrowingCapacity"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LineOfCreditFacilityLineItems_cd7ded0d-76bd-44ad-aa0a-2507b963167d" xlink:to="loc_ngs_LineOfCreditFacilityPotentialMaximumBorrowingCapacity_0496d7bc-208d-4b04-ae27-cb0cc5420885" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_ngs_LineOfCreditFacilityAmendmentFees_68bc5bac-024e-4fd3-8fb8-f2502cb90baa" xlink:href="ngs-20240331.xsd#ngs_LineOfCreditFacilityAmendmentFees"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LineOfCreditFacilityLineItems_cd7ded0d-76bd-44ad-aa0a-2507b963167d" xlink:to="loc_ngs_LineOfCreditFacilityAmendmentFees_68bc5bac-024e-4fd3-8fb8-f2502cb90baa" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_ngs_LineOfCreditFacilityAmendmentFeesPercentage_26620455-824a-46f5-bcad-e9d098f59efc" xlink:href="ngs-20240331.xsd#ngs_LineOfCreditFacilityAmendmentFeesPercentage"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LineOfCreditFacilityLineItems_cd7ded0d-76bd-44ad-aa0a-2507b963167d" xlink:to="loc_ngs_LineOfCreditFacilityAmendmentFeesPercentage_26620455-824a-46f5-bcad-e9d098f59efc" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LineOfCreditFacilityIncreaseDecreaseForPeriodNet_1167e46d-f5f7-42b1-ae47-533f9de1b09e" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_LineOfCreditFacilityIncreaseDecreaseForPeriodNet"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LineOfCreditFacilityLineItems_cd7ded0d-76bd-44ad-aa0a-2507b963167d" xlink:to="loc_us-gaap_LineOfCreditFacilityIncreaseDecreaseForPeriodNet_1167e46d-f5f7-42b1-ae47-533f9de1b09e" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LineOfCredit_b72fae4e-db24-4ed7-818d-4dcdfc2c3e13" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_LineOfCredit"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LineOfCreditFacilityLineItems_cd7ded0d-76bd-44ad-aa0a-2507b963167d" xlink:to="loc_us-gaap_LineOfCredit_b72fae4e-db24-4ed7-818d-4dcdfc2c3e13" xlink:type="arc" order="8"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongTermDebtWeightedAverageInterestRateOverTime_361a050b-422c-4c22-a100-1a6cc9e03a3b" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_LongTermDebtWeightedAverageInterestRateOverTime"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LineOfCreditFacilityLineItems_cd7ded0d-76bd-44ad-aa0a-2507b963167d" xlink:to="loc_us-gaap_LongTermDebtWeightedAverageInterestRateOverTime_361a050b-422c-4c22-a100-1a6cc9e03a3b" xlink:type="arc" order="9"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LineOfCreditFacilityRemainingBorrowingCapacity_255343db-b0fd-4a4e-93b8-8435d5557467" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_LineOfCreditFacilityRemainingBorrowingCapacity"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LineOfCreditFacilityLineItems_cd7ded0d-76bd-44ad-aa0a-2507b963167d" xlink:to="loc_us-gaap_LineOfCreditFacilityRemainingBorrowingCapacity_255343db-b0fd-4a4e-93b8-8435d5557467" xlink:type="arc" order="10"/>
    <link:loc xlink:type="locator" xlink:label="loc_ngs_LineOfCreditFacilityCurrentBorrowingCapacityComponentPercentOfEligibleAccountsReceivableOwingByNonInvestmentGradeDebtors_3837044e-313f-4fb6-8ee4-bb73c38cd70f" xlink:href="ngs-20240331.xsd#ngs_LineOfCreditFacilityCurrentBorrowingCapacityComponentPercentOfEligibleAccountsReceivableOwingByNonInvestmentGradeDebtors"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LineOfCreditFacilityLineItems_cd7ded0d-76bd-44ad-aa0a-2507b963167d" xlink:to="loc_ngs_LineOfCreditFacilityCurrentBorrowingCapacityComponentPercentOfEligibleAccountsReceivableOwingByNonInvestmentGradeDebtors_3837044e-313f-4fb6-8ee4-bb73c38cd70f" xlink:type="arc" order="11"/>
    <link:loc xlink:type="locator" xlink:label="loc_ngs_LineOfCreditFacilityCurrentBorrowingCapacityComponentPercentageOfEligibleInventory_8b567616-a5ba-4d34-9809-80e118d8b870" xlink:href="ngs-20240331.xsd#ngs_LineOfCreditFacilityCurrentBorrowingCapacityComponentPercentageOfEligibleInventory"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LineOfCreditFacilityLineItems_cd7ded0d-76bd-44ad-aa0a-2507b963167d" xlink:to="loc_ngs_LineOfCreditFacilityCurrentBorrowingCapacityComponentPercentageOfEligibleInventory_8b567616-a5ba-4d34-9809-80e118d8b870" xlink:type="arc" order="12"/>
    <link:loc xlink:type="locator" xlink:label="loc_ngs_LineOfCreditFacilityNonExceedingCapOfComponentCapacity_eeeb2228-2193-424e-8154-45ca456e5a05" xlink:href="ngs-20240331.xsd#ngs_LineOfCreditFacilityNonExceedingCapOfComponentCapacity"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LineOfCreditFacilityLineItems_cd7ded0d-76bd-44ad-aa0a-2507b963167d" xlink:to="loc_ngs_LineOfCreditFacilityNonExceedingCapOfComponentCapacity_eeeb2228-2193-424e-8154-45ca456e5a05" xlink:type="arc" order="13"/>
    <link:loc xlink:type="locator" xlink:label="loc_ngs_LineOfCreditFacilityCurrentBorrowingCapacityComponentPercentageOfEligibleCompressorsForExtensionOfCredit_3fa2c8f8-da0e-40a1-8326-2ed242dc3a19" xlink:href="ngs-20240331.xsd#ngs_LineOfCreditFacilityCurrentBorrowingCapacityComponentPercentageOfEligibleCompressorsForExtensionOfCredit"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LineOfCreditFacilityLineItems_cd7ded0d-76bd-44ad-aa0a-2507b963167d" xlink:to="loc_ngs_LineOfCreditFacilityCurrentBorrowingCapacityComponentPercentageOfEligibleCompressorsForExtensionOfCredit_3fa2c8f8-da0e-40a1-8326-2ed242dc3a19" xlink:type="arc" order="14"/>
    <link:loc xlink:type="locator" xlink:label="loc_ngs_LineOfCreditFacilityTermForDepreciation_4722976b-e1d6-4cc8-988a-b120527cd55c" xlink:href="ngs-20240331.xsd#ngs_LineOfCreditFacilityTermForDepreciation"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LineOfCreditFacilityLineItems_cd7ded0d-76bd-44ad-aa0a-2507b963167d" xlink:to="loc_ngs_LineOfCreditFacilityTermForDepreciation_4722976b-e1d6-4cc8-988a-b120527cd55c" xlink:type="arc" order="15"/>
    <link:loc xlink:type="locator" xlink:label="loc_ngs_LineOfCreditFacilityCurrentBorrowingCapacityNetLiquidationValueOfPercentageOfEligibleCompressorsForExtensionOfCredit_69963147-841f-4ee5-ab1a-8d32a960c4e6" xlink:href="ngs-20240331.xsd#ngs_LineOfCreditFacilityCurrentBorrowingCapacityNetLiquidationValueOfPercentageOfEligibleCompressorsForExtensionOfCredit"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LineOfCreditFacilityLineItems_cd7ded0d-76bd-44ad-aa0a-2507b963167d" xlink:to="loc_ngs_LineOfCreditFacilityCurrentBorrowingCapacityNetLiquidationValueOfPercentageOfEligibleCompressorsForExtensionOfCredit_69963147-841f-4ee5-ab1a-8d32a960c4e6" xlink:type="arc" order="16"/>
    <link:loc xlink:type="locator" xlink:label="loc_ngs_LineOfCreditFacilityCurrentBorrowingCapacityComponentPercentageOfEligibleValueAtCostExcludingAnyCostForCapitalizedInterestOrOtherNonCashCapitalizedCosts_4ab37dff-50d7-42ff-a49c-44d35536a64a" xlink:href="ngs-20240331.xsd#ngs_LineOfCreditFacilityCurrentBorrowingCapacityComponentPercentageOfEligibleValueAtCostExcludingAnyCostForCapitalizedInterestOrOtherNonCashCapitalizedCosts"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LineOfCreditFacilityLineItems_cd7ded0d-76bd-44ad-aa0a-2507b963167d" xlink:to="loc_ngs_LineOfCreditFacilityCurrentBorrowingCapacityComponentPercentageOfEligibleValueAtCostExcludingAnyCostForCapitalizedInterestOrOtherNonCashCapitalizedCosts_4ab37dff-50d7-42ff-a49c-44d35536a64a" xlink:type="arc" order="17"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentBasisSpreadOnVariableRate1_78fbf255-1094-4e30-a1c3-977092d5f056" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_DebtInstrumentBasisSpreadOnVariableRate1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LineOfCreditFacilityLineItems_cd7ded0d-76bd-44ad-aa0a-2507b963167d" xlink:to="loc_us-gaap_DebtInstrumentBasisSpreadOnVariableRate1_78fbf255-1094-4e30-a1c3-977092d5f056" xlink:type="arc" order="18"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LineOfCreditFacilityUnusedCapacityCommitmentFeePercentage_5bdf0dee-fbd6-47df-afe4-21aa1be0a97b" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_LineOfCreditFacilityUnusedCapacityCommitmentFeePercentage"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LineOfCreditFacilityLineItems_cd7ded0d-76bd-44ad-aa0a-2507b963167d" xlink:to="loc_us-gaap_LineOfCreditFacilityUnusedCapacityCommitmentFeePercentage_5bdf0dee-fbd6-47df-afe4-21aa1be0a97b" xlink:type="arc" order="19"/>
    <link:loc xlink:type="locator" xlink:label="loc_ngs_LineOfCreditFacilityLeverageRatio_c6515cb5-51eb-4cc2-b289-cbc1010af3a3" xlink:href="ngs-20240331.xsd#ngs_LineOfCreditFacilityLeverageRatio"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LineOfCreditFacilityLineItems_cd7ded0d-76bd-44ad-aa0a-2507b963167d" xlink:to="loc_ngs_LineOfCreditFacilityLeverageRatio_c6515cb5-51eb-4cc2-b289-cbc1010af3a3" xlink:type="arc" order="20"/>
    <link:loc xlink:type="locator" xlink:label="loc_ngs_LineOfCreditFacilityFixedChargeCoverageRatio_56b5c1e1-c84a-4e4d-8cfe-0713e2055c1d" xlink:href="ngs-20240331.xsd#ngs_LineOfCreditFacilityFixedChargeCoverageRatio"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LineOfCreditFacilityLineItems_cd7ded0d-76bd-44ad-aa0a-2507b963167d" xlink:to="loc_ngs_LineOfCreditFacilityFixedChargeCoverageRatio_56b5c1e1-c84a-4e4d-8cfe-0713e2055c1d" xlink:type="arc" order="21"/>
    <link:loc xlink:type="locator" xlink:label="loc_ngs_LineOfCreditFacilityDefaultTriggerCertainDefaultsOfOtherCompanyIndebtednessAmount_821cca42-03ea-44ed-8547-2bc84210cab0" xlink:href="ngs-20240331.xsd#ngs_LineOfCreditFacilityDefaultTriggerCertainDefaultsOfOtherCompanyIndebtednessAmount"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LineOfCreditFacilityLineItems_cd7ded0d-76bd-44ad-aa0a-2507b963167d" xlink:to="loc_ngs_LineOfCreditFacilityDefaultTriggerCertainDefaultsOfOtherCompanyIndebtednessAmount_821cca42-03ea-44ed-8547-2bc84210cab0" xlink:type="arc" order="22"/>
    <link:loc xlink:type="locator" xlink:label="loc_ngs_LineOfCreditFacilityDefaultTriggerRenderingOfCertainJudgmentsAmount_17b8ec64-32ec-4022-b62b-34b8f1c29410" xlink:href="ngs-20240331.xsd#ngs_LineOfCreditFacilityDefaultTriggerRenderingOfCertainJudgmentsAmount"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LineOfCreditFacilityLineItems_cd7ded0d-76bd-44ad-aa0a-2507b963167d" xlink:to="loc_ngs_LineOfCreditFacilityDefaultTriggerRenderingOfCertainJudgmentsAmount_17b8ec64-32ec-4022-b62b-34b8f1c29410" xlink:type="arc" order="23"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.ngsgi.com/role/StockBasedandOtherLongTermIncentiveCompensationScheduleofStockOptionsOutstandingDetails" xlink:type="simple" xlink:href="ngs-20240331.xsd#StockBasedandOtherLongTermIncentiveCompensationScheduleofStockOptionsOutstandingDetails"/>
  <link:definitionLink xlink:role="http://www.ngsgi.com/role/StockBasedandOtherLongTermIncentiveCompensationScheduleofStockOptionsOutstandingDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansExercisePriceRangeLineItems_f4d47680-4f1e-43a1-a5ac-eac7eec0749c" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansExercisePriceRangeLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfShareBasedCompensationSharesAuthorizedUnderStockOptionPlansByExercisePriceRangeTable_ecefe826-f513-470e-9f23-aa69873decec" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_ScheduleOfShareBasedCompensationSharesAuthorizedUnderStockOptionPlansByExercisePriceRangeTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansExercisePriceRangeLineItems_f4d47680-4f1e-43a1-a5ac-eac7eec0749c" xlink:to="loc_us-gaap_ScheduleOfShareBasedCompensationSharesAuthorizedUnderStockOptionPlansByExercisePriceRangeTable_ecefe826-f513-470e-9f23-aa69873decec" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansByExercisePriceRangeAxis_173c3b30-a628-47e7-9897-1fb3ffb54380" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansByExercisePriceRangeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfShareBasedCompensationSharesAuthorizedUnderStockOptionPlansByExercisePriceRangeTable_ecefe826-f513-470e-9f23-aa69873decec" xlink:to="loc_us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansByExercisePriceRangeAxis_173c3b30-a628-47e7-9897-1fb3ffb54380" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansExercisePriceRangeDomain_173c3b30-a628-47e7-9897-1fb3ffb54380_default" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansExercisePriceRangeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansByExercisePriceRangeAxis_173c3b30-a628-47e7-9897-1fb3ffb54380" xlink:to="loc_us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansExercisePriceRangeDomain_173c3b30-a628-47e7-9897-1fb3ffb54380_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansExercisePriceRangeDomain_dfd67b8c-37fd-4689-a757-f1cb5a446793" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansExercisePriceRangeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansByExercisePriceRangeAxis_173c3b30-a628-47e7-9897-1fb3ffb54380" xlink:to="loc_us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansExercisePriceRangeDomain_dfd67b8c-37fd-4689-a757-f1cb5a446793" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_ngs_RangeOneMember_69ed6078-7f20-40d6-9e8f-472c05ba3a39" xlink:href="ngs-20240331.xsd#ngs_RangeOneMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansExercisePriceRangeDomain_dfd67b8c-37fd-4689-a757-f1cb5a446793" xlink:to="loc_ngs_RangeOneMember_69ed6078-7f20-40d6-9e8f-472c05ba3a39" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_ngs_RangeTwoMember_f65c9793-2903-4654-a8dd-83e4f39ddc0d" xlink:href="ngs-20240331.xsd#ngs_RangeTwoMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansExercisePriceRangeDomain_dfd67b8c-37fd-4689-a757-f1cb5a446793" xlink:to="loc_ngs_RangeTwoMember_f65c9793-2903-4654-a8dd-83e4f39ddc0d" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_ngs_RangeThreeMember_59740c13-c626-4e2f-8beb-c74297464323" xlink:href="ngs-20240331.xsd#ngs_RangeThreeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansExercisePriceRangeDomain_dfd67b8c-37fd-4689-a757-f1cb5a446793" xlink:to="loc_ngs_RangeThreeMember_59740c13-c626-4e2f-8beb-c74297464323" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansExercisePriceRangeLowerRangeLimit_9860f74d-c18f-4292-8e00-ef3484c6af74" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansExercisePriceRangeLowerRangeLimit"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansExercisePriceRangeLineItems_f4d47680-4f1e-43a1-a5ac-eac7eec0749c" xlink:to="loc_us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansExercisePriceRangeLowerRangeLimit_9860f74d-c18f-4292-8e00-ef3484c6af74" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansExercisePriceRangeUpperRangeLimit_afae058d-b4a0-4368-84bc-cac03f942774" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansExercisePriceRangeUpperRangeLimit"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansExercisePriceRangeLineItems_f4d47680-4f1e-43a1-a5ac-eac7eec0749c" xlink:to="loc_us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansExercisePriceRangeUpperRangeLimit_afae058d-b4a0-4368-84bc-cac03f942774" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansExercisePriceRangeNumberOfOutstandingOptions_918857cd-834e-4556-97a7-2c757d8e0384" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansExercisePriceRangeNumberOfOutstandingOptions"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansExercisePriceRangeLineItems_f4d47680-4f1e-43a1-a5ac-eac7eec0749c" xlink:to="loc_us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansExercisePriceRangeNumberOfOutstandingOptions_918857cd-834e-4556-97a7-2c757d8e0384" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SharebasedCompensationSharesAuthorizedUnderStockOptionPlansExercisePriceRangeOutstandingOptionsWeightedAverageRemainingContractualTerm2_d111e671-74be-4e5c-8e58-a66d9b3878a8" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_SharebasedCompensationSharesAuthorizedUnderStockOptionPlansExercisePriceRangeOutstandingOptionsWeightedAverageRemainingContractualTerm2"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansExercisePriceRangeLineItems_f4d47680-4f1e-43a1-a5ac-eac7eec0749c" xlink:to="loc_us-gaap_SharebasedCompensationSharesAuthorizedUnderStockOptionPlansExercisePriceRangeOutstandingOptionsWeightedAverageRemainingContractualTerm2_d111e671-74be-4e5c-8e58-a66d9b3878a8" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SharebasedCompensationSharesAuthorizedUnderStockOptionPlansExercisePriceRangeOutstandingOptionsWeightedAverageExercisePriceBeginningBalance1_a5d5eece-654c-4470-92ba-e3517c3a41b8" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_SharebasedCompensationSharesAuthorizedUnderStockOptionPlansExercisePriceRangeOutstandingOptionsWeightedAverageExercisePriceBeginningBalance1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansExercisePriceRangeLineItems_f4d47680-4f1e-43a1-a5ac-eac7eec0749c" xlink:to="loc_us-gaap_SharebasedCompensationSharesAuthorizedUnderStockOptionPlansExercisePriceRangeOutstandingOptionsWeightedAverageExercisePriceBeginningBalance1_a5d5eece-654c-4470-92ba-e3517c3a41b8" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansExercisePriceRangeNumberOfExercisableOptions_5ca65812-53b2-45de-9890-79aa84227a6b" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansExercisePriceRangeNumberOfExercisableOptions"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansExercisePriceRangeLineItems_f4d47680-4f1e-43a1-a5ac-eac7eec0749c" xlink:to="loc_us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansExercisePriceRangeNumberOfExercisableOptions_5ca65812-53b2-45de-9890-79aa84227a6b" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SharebasedCompensationSharesAuthorizedUnderStockOptionPlansExercisePriceRangeExercisableOptionsWeightedAverageExercisePrice1_f5000f29-e4ce-4f9c-9eb6-1f843bf07fee" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_SharebasedCompensationSharesAuthorizedUnderStockOptionPlansExercisePriceRangeExercisableOptionsWeightedAverageExercisePrice1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansExercisePriceRangeLineItems_f4d47680-4f1e-43a1-a5ac-eac7eec0749c" xlink:to="loc_us-gaap_SharebasedCompensationSharesAuthorizedUnderStockOptionPlansExercisePriceRangeExercisableOptionsWeightedAverageExercisePrice1_f5000f29-e4ce-4f9c-9eb6-1f843bf07fee" xlink:type="arc" order="6"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.ngsgi.com/role/StockBasedandOtherLongTermIncentiveCompensationStockOptionNarrativeDetails" xlink:type="simple" xlink:href="ngs-20240331.xsd#StockBasedandOtherLongTermIncentiveCompensationStockOptionNarrativeDetails"/>
  <link:definitionLink xlink:role="http://www.ngsgi.com/role/StockBasedandOtherLongTermIncentiveCompensationStockOptionNarrativeDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_cc70e5e8-18db-4d9a-ac6f-55001733c3eb" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_2a2956db-0b06-45bd-9e35-402c4c1055d7" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_cc70e5e8-18db-4d9a-ac6f-55001733c3eb" xlink:to="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_2a2956db-0b06-45bd-9e35-402c4c1055d7" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AwardTypeAxis_995e588c-a546-4947-aaa3-daae6236ff2e" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_AwardTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_2a2956db-0b06-45bd-9e35-402c4c1055d7" xlink:to="loc_us-gaap_AwardTypeAxis_995e588c-a546-4947-aaa3-daae6236ff2e" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_995e588c-a546-4947-aaa3-daae6236ff2e_default" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_AwardTypeAxis_995e588c-a546-4947-aaa3-daae6236ff2e" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_995e588c-a546-4947-aaa3-daae6236ff2e_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_cfc20cad-aa1e-499d-86d6-c69ff08acc0d" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_AwardTypeAxis_995e588c-a546-4947-aaa3-daae6236ff2e" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_cfc20cad-aa1e-499d-86d6-c69ff08acc0d" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EmployeeStockOptionMember_d4e32b6f-6596-4bdf-a3be-7ad2c3e254f0" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_EmployeeStockOptionMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_cfc20cad-aa1e-499d-86d6-c69ff08acc0d" xlink:to="loc_us-gaap_EmployeeStockOptionMember_d4e32b6f-6596-4bdf-a3be-7ad2c3e254f0" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognized_01704aa1-95f6-47ac-b649-364cba5e012c" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognized"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_cc70e5e8-18db-4d9a-ac6f-55001733c3eb" xlink:to="loc_us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognized_01704aa1-95f6-47ac-b649-364cba5e012c" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AllocatedShareBasedCompensationExpense_a3021354-ef7d-481b-8fd8-8ba052f10c7d" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_AllocatedShareBasedCompensationExpense"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_cc70e5e8-18db-4d9a-ac6f-55001733c3eb" xlink:to="loc_us-gaap_AllocatedShareBasedCompensationExpense_a3021354-ef7d-481b-8fd8-8ba052f10c7d" xlink:type="arc" order="1"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.ngsgi.com/role/StockBasedandOtherLongTermIncentiveCompensationRestrictedSharesUnitsNarrativeDetails" xlink:type="simple" xlink:href="ngs-20240331.xsd#StockBasedandOtherLongTermIncentiveCompensationRestrictedSharesUnitsNarrativeDetails"/>
  <link:definitionLink xlink:role="http://www.ngsgi.com/role/StockBasedandOtherLongTermIncentiveCompensationRestrictedSharesUnitsNarrativeDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_4a9e40aa-e1ce-4948-a509-f8360f717675" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_069e409f-c8c8-44e7-baa4-2f2336fd223a" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_4a9e40aa-e1ce-4948-a509-f8360f717675" xlink:to="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_069e409f-c8c8-44e7-baa4-2f2336fd223a" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AwardTypeAxis_7411967b-10b1-4a66-94a4-84afdbb263fa" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_AwardTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_069e409f-c8c8-44e7-baa4-2f2336fd223a" xlink:to="loc_us-gaap_AwardTypeAxis_7411967b-10b1-4a66-94a4-84afdbb263fa" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_7411967b-10b1-4a66-94a4-84afdbb263fa_default" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_AwardTypeAxis_7411967b-10b1-4a66-94a4-84afdbb263fa" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_7411967b-10b1-4a66-94a4-84afdbb263fa_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_ce64c4b4-4333-426c-90f2-143c5c6f813c" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_AwardTypeAxis_7411967b-10b1-4a66-94a4-84afdbb263fa" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_ce64c4b4-4333-426c-90f2-143c5c6f813c" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RestrictedStockMember_e8ffbcb7-9d74-4ab5-ba49-14d3e1c6fbe9" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_RestrictedStockMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_ce64c4b4-4333-426c-90f2-143c5c6f813c" xlink:to="loc_us-gaap_RestrictedStockMember_e8ffbcb7-9d74-4ab5-ba49-14d3e1c6fbe9" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RestrictedStockUnitsRSUMember_a4caaa1c-534b-46f4-b9bf-ef648e74fcda" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_RestrictedStockUnitsRSUMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_ce64c4b4-4333-426c-90f2-143c5c6f813c" xlink:to="loc_us-gaap_RestrictedStockUnitsRSUMember_a4caaa1c-534b-46f4-b9bf-ef648e74fcda" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonStockMember_23835df2-801d-46a7-987d-6957d5f13413" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_CommonStockMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_ce64c4b4-4333-426c-90f2-143c5c6f813c" xlink:to="loc_us-gaap_CommonStockMember_23835df2-801d-46a7-987d-6957d5f13413" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_ngs_PerformanceSharesSigningBonusMember_6b0960d4-4550-47dc-b18f-d73b9df02d4c" xlink:href="ngs-20240331.xsd#ngs_PerformanceSharesSigningBonusMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_ce64c4b4-4333-426c-90f2-143c5c6f813c" xlink:to="loc_ngs_PerformanceSharesSigningBonusMember_6b0960d4-4550-47dc-b18f-d73b9df02d4c" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_ngs_PerformanceShares2024ProRatedMember_b424133a-ea9b-4ccd-836d-b356a70f7e82" xlink:href="ngs-20240331.xsd#ngs_PerformanceShares2024ProRatedMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_ce64c4b4-4333-426c-90f2-143c5c6f813c" xlink:to="loc_ngs_PerformanceShares2024ProRatedMember_b424133a-ea9b-4ccd-836d-b356a70f7e82" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PerformanceSharesMember_318fb1bd-3369-482f-b42b-43894c53e3c5" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_PerformanceSharesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_ce64c4b4-4333-426c-90f2-143c5c6f813c" xlink:to="loc_us-gaap_PerformanceSharesMember_318fb1bd-3369-482f-b42b-43894c53e3c5" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_TitleOfIndividualAxis_a91aa422-5842-474c-9a66-5732d905f78b" xlink:href="https://xbrl.fasb.org/srt/2023/elts/srt-2023.xsd#srt_TitleOfIndividualAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_069e409f-c8c8-44e7-baa4-2f2336fd223a" xlink:to="loc_srt_TitleOfIndividualAxis_a91aa422-5842-474c-9a66-5732d905f78b" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_TitleOfIndividualWithRelationshipToEntityDomain_a91aa422-5842-474c-9a66-5732d905f78b_default" xlink:href="https://xbrl.fasb.org/srt/2023/elts/srt-2023.xsd#srt_TitleOfIndividualWithRelationshipToEntityDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_TitleOfIndividualAxis_a91aa422-5842-474c-9a66-5732d905f78b" xlink:to="loc_srt_TitleOfIndividualWithRelationshipToEntityDomain_a91aa422-5842-474c-9a66-5732d905f78b_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_TitleOfIndividualWithRelationshipToEntityDomain_49e840b9-350a-4c60-8b35-dce56248e836" xlink:href="https://xbrl.fasb.org/srt/2023/elts/srt-2023.xsd#srt_TitleOfIndividualWithRelationshipToEntityDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_TitleOfIndividualAxis_a91aa422-5842-474c-9a66-5732d905f78b" xlink:to="loc_srt_TitleOfIndividualWithRelationshipToEntityDomain_49e840b9-350a-4c60-8b35-dce56248e836" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_ngs_ChiefTechnicalOfficerMember_2b456bac-00d8-49ae-9e4e-d36546bf72d7" xlink:href="ngs-20240331.xsd#ngs_ChiefTechnicalOfficerMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_TitleOfIndividualWithRelationshipToEntityDomain_49e840b9-350a-4c60-8b35-dce56248e836" xlink:to="loc_ngs_ChiefTechnicalOfficerMember_2b456bac-00d8-49ae-9e4e-d36546bf72d7" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ChiefExecutiveOfficerMember_6fc27f21-b0fc-4c34-9b34-d2b8464f18f7" xlink:href="https://xbrl.fasb.org/srt/2023/elts/srt-2023.xsd#srt_ChiefExecutiveOfficerMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_TitleOfIndividualWithRelationshipToEntityDomain_49e840b9-350a-4c60-8b35-dce56248e836" xlink:to="loc_srt_ChiefExecutiveOfficerMember_6fc27f21-b0fc-4c34-9b34-d2b8464f18f7" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_DirectorMember_769e724a-5383-45df-b683-426113147534" xlink:href="https://xbrl.fasb.org/srt/2023/elts/srt-2023.xsd#srt_DirectorMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_TitleOfIndividualWithRelationshipToEntityDomain_49e840b9-350a-4c60-8b35-dce56248e836" xlink:to="loc_srt_DirectorMember_769e724a-5383-45df-b683-426113147534" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ChiefOperatingOfficerMember_c6487cc7-694d-432f-be05-c2cf6c64054b" xlink:href="https://xbrl.fasb.org/srt/2023/elts/srt-2023.xsd#srt_ChiefOperatingOfficerMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_TitleOfIndividualWithRelationshipToEntityDomain_49e840b9-350a-4c60-8b35-dce56248e836" xlink:to="loc_srt_ChiefOperatingOfficerMember_c6487cc7-694d-432f-be05-c2cf6c64054b" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_ngs_ChiefOperatingOfficerAndChiefExecutiveOfficerMember_4169cfda-58cf-4742-9d1b-04879b7e83d6" xlink:href="ngs-20240331.xsd#ngs_ChiefOperatingOfficerAndChiefExecutiveOfficerMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_TitleOfIndividualWithRelationshipToEntityDomain_49e840b9-350a-4c60-8b35-dce56248e836" xlink:to="loc_ngs_ChiefOperatingOfficerAndChiefExecutiveOfficerMember_4169cfda-58cf-4742-9d1b-04879b7e83d6" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeAxis_524d788f-26c0-4ab4-8866-8f18e2f31235" xlink:href="https://xbrl.fasb.org/srt/2023/elts/srt-2023.xsd#srt_RangeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_069e409f-c8c8-44e7-baa4-2f2336fd223a" xlink:to="loc_srt_RangeAxis_524d788f-26c0-4ab4-8866-8f18e2f31235" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeMember_524d788f-26c0-4ab4-8866-8f18e2f31235_default" xlink:href="https://xbrl.fasb.org/srt/2023/elts/srt-2023.xsd#srt_RangeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_RangeAxis_524d788f-26c0-4ab4-8866-8f18e2f31235" xlink:to="loc_srt_RangeMember_524d788f-26c0-4ab4-8866-8f18e2f31235_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeMember_86787948-0bbf-4017-85f8-6d45b3b42dfb" xlink:href="https://xbrl.fasb.org/srt/2023/elts/srt-2023.xsd#srt_RangeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_RangeAxis_524d788f-26c0-4ab4-8866-8f18e2f31235" xlink:to="loc_srt_RangeMember_86787948-0bbf-4017-85f8-6d45b3b42dfb" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_MinimumMember_3829e874-45d1-45fc-bf3a-3b44e021121e" xlink:href="https://xbrl.fasb.org/srt/2023/elts/srt-2023.xsd#srt_MinimumMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RangeMember_86787948-0bbf-4017-85f8-6d45b3b42dfb" xlink:to="loc_srt_MinimumMember_3829e874-45d1-45fc-bf3a-3b44e021121e" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_MaximumMember_f2908587-f14e-4471-90db-07974c158b78" xlink:href="https://xbrl.fasb.org/srt/2023/elts/srt-2023.xsd#srt_MaximumMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RangeMember_86787948-0bbf-4017-85f8-6d45b3b42dfb" xlink:to="loc_srt_MaximumMember_f2908587-f14e-4471-90db-07974c158b78" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod_93680f11-0a5e-4646-addb-bfd09c17d7e8" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_4a9e40aa-e1ce-4948-a509-f8360f717675" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod_93680f11-0a5e-4646-addb-bfd09c17d7e8" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardAwardVestingPeriod1_bf12bebe-bc26-48bf-8c50-e95a450f51b9" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardAwardVestingPeriod1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_4a9e40aa-e1ce-4948-a509-f8360f717675" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardAwardVestingPeriod1_bf12bebe-bc26-48bf-8c50-e95a450f51b9" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_ngs_NumberOfIndependentDirectors_35ab15bd-6869-4442-a08b-5ef96f1e79df" xlink:href="ngs-20240331.xsd#ngs_NumberOfIndependentDirectors"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_4a9e40aa-e1ce-4948-a509-f8360f717675" xlink:to="loc_ngs_NumberOfIndependentDirectors_35ab15bd-6869-4442-a08b-5ef96f1e79df" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsForfeitedInPeriod_3a660d7e-8f93-4285-affe-2eab5931f532" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsForfeitedInPeriod"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_4a9e40aa-e1ce-4948-a509-f8360f717675" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsForfeitedInPeriod_3a660d7e-8f93-4285-affe-2eab5931f532" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_ngs_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedNumberOfSharesToBeTransferred_c03a7f0b-cfcf-4e15-8567-2402850ceb00" xlink:href="ngs-20240331.xsd#ngs_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedNumberOfSharesToBeTransferred"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_4a9e40aa-e1ce-4948-a509-f8360f717675" xlink:to="loc_ngs_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedNumberOfSharesToBeTransferred_c03a7f0b-cfcf-4e15-8567-2402850ceb00" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriod_ddaea57c-8c10-48fd-9d69-db9fa9bc53b6" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriod"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_4a9e40aa-e1ce-4948-a509-f8360f717675" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriod_ddaea57c-8c10-48fd-9d69-db9fa9bc53b6" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AllocatedShareBasedCompensationExpense_a25f8bce-3456-45b8-84e7-d23868b4c26a" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_AllocatedShareBasedCompensationExpense"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_4a9e40aa-e1ce-4948-a509-f8360f717675" xlink:to="loc_us-gaap_AllocatedShareBasedCompensationExpense_a25f8bce-3456-45b8-84e7-d23868b4c26a" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardAwardRequisiteServicePeriod1_c85b8d2e-aadb-4509-875f-59ef8a96371f" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardAwardRequisiteServicePeriod1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_4a9e40aa-e1ce-4948-a509-f8360f717675" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardAwardRequisiteServicePeriod1_c85b8d2e-aadb-4509-875f-59ef8a96371f" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardAwardVestingRightsPercentage_c122fba5-d36b-4e10-9a1d-21b27d458a91" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardAwardVestingRightsPercentage"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_4a9e40aa-e1ce-4948-a509-f8360f717675" xlink:to="loc_us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardAwardVestingRightsPercentage_c122fba5-d36b-4e10-9a1d-21b27d458a91" xlink:type="arc" order="8"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.ngsgi.com/role/StockBasedandOtherLongTermIncentiveCompensationScheduleofRestrictedStockActivityDetails" xlink:type="simple" xlink:href="ngs-20240331.xsd#StockBasedandOtherLongTermIncentiveCompensationScheduleofRestrictedStockActivityDetails"/>
  <link:definitionLink xlink:role="http://www.ngsgi.com/role/StockBasedandOtherLongTermIncentiveCompensationScheduleofRestrictedStockActivityDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_55a1e351-2d77-4932-b74d-2eea0c195085" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_54421e85-4328-4cd5-91b6-719e1593c6c8" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_55a1e351-2d77-4932-b74d-2eea0c195085" xlink:to="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_54421e85-4328-4cd5-91b6-719e1593c6c8" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AwardTypeAxis_57b3c576-eaf2-439e-b5cd-2646298e5e49" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_AwardTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_54421e85-4328-4cd5-91b6-719e1593c6c8" xlink:to="loc_us-gaap_AwardTypeAxis_57b3c576-eaf2-439e-b5cd-2646298e5e49" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_57b3c576-eaf2-439e-b5cd-2646298e5e49_default" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_AwardTypeAxis_57b3c576-eaf2-439e-b5cd-2646298e5e49" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_57b3c576-eaf2-439e-b5cd-2646298e5e49_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_5cd5e8ba-f379-40f7-ba9d-cac0c432c636" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_AwardTypeAxis_57b3c576-eaf2-439e-b5cd-2646298e5e49" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_5cd5e8ba-f379-40f7-ba9d-cac0c432c636" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RestrictedStockMember_606faad3-ede0-4ac3-a641-506ee3c474d4" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_RestrictedStockMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_5cd5e8ba-f379-40f7-ba9d-cac0c432c636" xlink:to="loc_us-gaap_RestrictedStockMember_606faad3-ede0-4ac3-a641-506ee3c474d4" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedRollForward_7adeabfd-f76b-4ca8-ada1-1145045337d3" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedRollForward"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_55a1e351-2d77-4932-b74d-2eea0c195085" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedRollForward_7adeabfd-f76b-4ca8-ada1-1145045337d3" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber_c184263f-612c-40ea-9a0c-8b082a52d148" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedRollForward_7adeabfd-f76b-4ca8-ada1-1145045337d3" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber_c184263f-612c-40ea-9a0c-8b082a52d148" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod_963a2610-ad95-4b18-9ae5-9fc1e0d3bc70" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedRollForward_7adeabfd-f76b-4ca8-ada1-1145045337d3" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod_963a2610-ad95-4b18-9ae5-9fc1e0d3bc70" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriod_8f58dc1c-a8c5-4bc7-bc1b-5c2e97a5d47f" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriod"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedRollForward_7adeabfd-f76b-4ca8-ada1-1145045337d3" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriod_8f58dc1c-a8c5-4bc7-bc1b-5c2e97a5d47f" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsForfeitedInPeriod_28d2402e-a986-4a48-b635-c334931ce1a3" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsForfeitedInPeriod"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedRollForward_7adeabfd-f76b-4ca8-ada1-1145045337d3" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsForfeitedInPeriod_28d2402e-a986-4a48-b635-c334931ce1a3" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber_330b0739-17ed-4665-9496-046f590988e1" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValueRollForward_ebd69118-294f-4ad7-8b2a-a7a1a40fd4ad" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValueRollForward"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_55a1e351-2d77-4932-b74d-2eea0c195085" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValueRollForward_ebd69118-294f-4ad7-8b2a-a7a1a40fd4ad" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValue_da01ea1c-1cef-4919-a216-b12f69190fbb" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValueRollForward_ebd69118-294f-4ad7-8b2a-a7a1a40fd4ad" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValue_da01ea1c-1cef-4919-a216-b12f69190fbb" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriodWeightedAverageGrantDateFairValue_2c47fb38-6a07-4d11-8c91-c4a5bc7f190d" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriodWeightedAverageGrantDateFairValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValueRollForward_ebd69118-294f-4ad7-8b2a-a7a1a40fd4ad" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriodWeightedAverageGrantDateFairValue_2c47fb38-6a07-4d11-8c91-c4a5bc7f190d" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriodWeightedAverageGrantDateFairValue_4ae22717-714d-4ef4-957d-258555f615c0" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriodWeightedAverageGrantDateFairValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValueRollForward_ebd69118-294f-4ad7-8b2a-a7a1a40fd4ad" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriodWeightedAverageGrantDateFairValue_4ae22717-714d-4ef4-957d-258555f615c0" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsForfeituresWeightedAverageGrantDateFairValue_2077922f-7999-4a39-8f8f-5833c1b97bd1" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsForfeituresWeightedAverageGrantDateFairValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValueRollForward_ebd69118-294f-4ad7-8b2a-a7a1a40fd4ad" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsForfeituresWeightedAverageGrantDateFairValue_2077922f-7999-4a39-8f8f-5833c1b97bd1" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValue_edf7cbe0-e006-48de-8fb4-9e6e5c3f690b" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValue"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsAdditionalDisclosuresAbstract_0fae05eb-46ec-4e0d-989a-cdb97b07edea" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsAdditionalDisclosuresAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_55a1e351-2d77-4932-b74d-2eea0c195085" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsAdditionalDisclosuresAbstract_0fae05eb-46ec-4e0d-989a-cdb97b07edea" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsOutstandingWeightedAverageRemainingContractualTerms_9acf9ea5-fa54-43b2-a8f0-4172ee80f6b5" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsOutstandingWeightedAverageRemainingContractualTerms"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsAdditionalDisclosuresAbstract_0fae05eb-46ec-4e0d-989a-cdb97b07edea" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsOutstandingWeightedAverageRemainingContractualTerms_9acf9ea5-fa54-43b2-a8f0-4172ee80f6b5" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardEquityInstrumentsOtherThanOptionsAggregateIntrinsicValueAbstract_991131f9-81ff-40f7-9078-1a7ee6bc6783" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardEquityInstrumentsOtherThanOptionsAggregateIntrinsicValueAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_55a1e351-2d77-4932-b74d-2eea0c195085" xlink:to="loc_us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardEquityInstrumentsOtherThanOptionsAggregateIntrinsicValueAbstract_991131f9-81ff-40f7-9078-1a7ee6bc6783" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardEquityInstrumentsOtherThanOptionsAggregateIntrinsicValueOutstanding_17056033-acc4-4b89-8a75-367dbcd5842c" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardEquityInstrumentsOtherThanOptionsAggregateIntrinsicValueOutstanding"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardEquityInstrumentsOtherThanOptionsAggregateIntrinsicValueAbstract_991131f9-81ff-40f7-9078-1a7ee6bc6783" xlink:to="loc_us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardEquityInstrumentsOtherThanOptionsAggregateIntrinsicValueOutstanding_17056033-acc4-4b89-8a75-367dbcd5842c" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_ngs_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsAggregateIntrinsicValueGrantsInPeriod_17ae8e75-259c-4c4c-bc50-a740cfa300c3" xlink:href="ngs-20240331.xsd#ngs_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsAggregateIntrinsicValueGrantsInPeriod"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardEquityInstrumentsOtherThanOptionsAggregateIntrinsicValueAbstract_991131f9-81ff-40f7-9078-1a7ee6bc6783" xlink:to="loc_ngs_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsAggregateIntrinsicValueGrantsInPeriod_17ae8e75-259c-4c4c-bc50-a740cfa300c3" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardEquityInstrumentsOtherThanOptionsAggregateIntrinsicValueVested_fe550ade-f23b-40a8-b365-6f4fff0409bc" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardEquityInstrumentsOtherThanOptionsAggregateIntrinsicValueVested"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardEquityInstrumentsOtherThanOptionsAggregateIntrinsicValueAbstract_991131f9-81ff-40f7-9078-1a7ee6bc6783" xlink:to="loc_us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardEquityInstrumentsOtherThanOptionsAggregateIntrinsicValueVested_fe550ade-f23b-40a8-b365-6f4fff0409bc" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_ngs_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsCancelledOrForfeitedInPeriodIntrinsicValue_6db5e1cc-8bfe-4835-877a-189543779712" xlink:href="ngs-20240331.xsd#ngs_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsCancelledOrForfeitedInPeriodIntrinsicValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardEquityInstrumentsOtherThanOptionsAggregateIntrinsicValueAbstract_991131f9-81ff-40f7-9078-1a7ee6bc6783" xlink:to="loc_ngs_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsCancelledOrForfeitedInPeriodIntrinsicValue_6db5e1cc-8bfe-4835-877a-189543779712" xlink:type="arc" order="4"/>
  </link:definitionLink>
</link:linkbase>
