<?xml version='1.0' encoding='utf-8'?>
<FilingSummary>
  <Version>3.23.2</Version>
  <ProcessingTime/>
  <ReportFormat>html</ReportFormat>
  <ContextCount>702</ContextCount>
  <ElementCount>298</ElementCount>
  <EntityCount>1</EntityCount>
  <FootnotesReported>true</FootnotesReported>
  <SegmentCount>110</SegmentCount>
  <ScenarioCount>0</ScenarioCount>
  <TuplesReported>false</TuplesReported>
  <UnitCount>7</UnitCount>
  <MyReports>
    <Report instance="gain-20230630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R1.htm</HtmlFileName>
      <LongName>0000001 - Document - Cover</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.gladstoneinvestment.com/role/Cover</Role>
      <ShortName>Cover</ShortName>
      <MenuCategory>Cover</MenuCategory>
      <Position>1</Position>
    </Report>
    <Report instance="gain-20230630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R2.htm</HtmlFileName>
      <LongName>0000002 - Statement - CONSOLIDATED STATEMENTS OF ASSETS AND LIABILITIES</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.gladstoneinvestment.com/role/CONSOLIDATEDSTATEMENTSOFASSETSANDLIABILITIES</Role>
      <ShortName>CONSOLIDATED STATEMENTS OF ASSETS AND LIABILITIES</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>2</Position>
    </Report>
    <Report instance="gain-20230630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R3.htm</HtmlFileName>
      <LongName>0000003 - Statement - CONSOLIDATED STATEMENTS OF ASSETS AND LIABILITIES (Parenthetical)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.gladstoneinvestment.com/role/CONSOLIDATEDSTATEMENTSOFASSETSANDLIABILITIESParenthetical</Role>
      <ShortName>CONSOLIDATED STATEMENTS OF ASSETS AND LIABILITIES (Parenthetical)</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>3</Position>
    </Report>
    <Report instance="gain-20230630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R4.htm</HtmlFileName>
      <LongName>0000004 - Statement - CONSOLIDATED STATEMENTS OF OPERATIONS</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.gladstoneinvestment.com/role/CONSOLIDATEDSTATEMENTSOFOPERATIONS</Role>
      <ShortName>CONSOLIDATED STATEMENTS OF OPERATIONS</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>4</Position>
    </Report>
    <Report instance="gain-20230630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R5.htm</HtmlFileName>
      <LongName>0000005 - Statement - CONSOLIDATED STATEMENTS OF CHANGES IN NET ASSETS</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.gladstoneinvestment.com/role/CONSOLIDATEDSTATEMENTSOFCHANGESINNETASSETS</Role>
      <ShortName>CONSOLIDATED STATEMENTS OF CHANGES IN NET ASSETS</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>5</Position>
    </Report>
    <Report instance="gain-20230630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R6.htm</HtmlFileName>
      <LongName>0000006 - Statement - CONSOLIDATED STATEMENTS OF CHANGES IN NET ASSETS (Parentheticals)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.gladstoneinvestment.com/role/CONSOLIDATEDSTATEMENTSOFCHANGESINNETASSETSParentheticals</Role>
      <ShortName>CONSOLIDATED STATEMENTS OF CHANGES IN NET ASSETS (Parentheticals)</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>6</Position>
    </Report>
    <Report instance="gain-20230630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R7.htm</HtmlFileName>
      <LongName>0000007 - Statement - CONSOLIDATED STATEMENTS OF CASH FLOWS</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.gladstoneinvestment.com/role/CONSOLIDATEDSTATEMENTSOFCASHFLOWS</Role>
      <ShortName>CONSOLIDATED STATEMENTS OF CASH FLOWS</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>7</Position>
    </Report>
    <Report instance="gain-20230630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R8.htm</HtmlFileName>
      <LongName>0000008 - Statement - CONSOLIDATED SCHEDULE OF INVESTMENTS</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.gladstoneinvestment.com/role/CONSOLIDATEDSCHEDULEOFINVESTMENTS</Role>
      <ShortName>CONSOLIDATED SCHEDULE OF INVESTMENTS</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>8</Position>
    </Report>
    <Report instance="gain-20230630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R9.htm</HtmlFileName>
      <LongName>0000009 - Statement - CONSOLIDATED SCHEDULE OF INVESTMENTS (Parentheticals)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.gladstoneinvestment.com/role/CONSOLIDATEDSCHEDULEOFINVESTMENTSParentheticals</Role>
      <ShortName>CONSOLIDATED SCHEDULE OF INVESTMENTS (Parentheticals)</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>9</Position>
    </Report>
    <Report instance="gain-20230630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R10.htm</HtmlFileName>
      <LongName>0000010 - Disclosure - ORGANIZATION</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.gladstoneinvestment.com/role/ORGANIZATION</Role>
      <ShortName>ORGANIZATION</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>10</Position>
    </Report>
    <Report instance="gain-20230630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R11.htm</HtmlFileName>
      <LongName>0000011 - Disclosure - SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.gladstoneinvestment.com/role/SUMMARYOFSIGNIFICANTACCOUNTINGPOLICIES</Role>
      <ShortName>SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>11</Position>
    </Report>
    <Report instance="gain-20230630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R12.htm</HtmlFileName>
      <LongName>0000012 - Disclosure - INVESTMENTS</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.gladstoneinvestment.com/role/INVESTMENTS</Role>
      <ShortName>INVESTMENTS</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>12</Position>
    </Report>
    <Report instance="gain-20230630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R13.htm</HtmlFileName>
      <LongName>0000013 - Disclosure - RELATED PARTY TRANSACTIONS</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.gladstoneinvestment.com/role/RELATEDPARTYTRANSACTIONS</Role>
      <ShortName>RELATED PARTY TRANSACTIONS</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>13</Position>
    </Report>
    <Report instance="gain-20230630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R14.htm</HtmlFileName>
      <LongName>0000014 - Disclosure - BORROWINGS</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.gladstoneinvestment.com/role/BORROWINGS</Role>
      <ShortName>BORROWINGS</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>14</Position>
    </Report>
    <Report instance="gain-20230630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R15.htm</HtmlFileName>
      <LongName>0000015 - Disclosure - REGISTRATION STATEMENT AND COMMON EQUITY OFFERINGS</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.gladstoneinvestment.com/role/REGISTRATIONSTATEMENTANDCOMMONEQUITYOFFERINGS</Role>
      <ShortName>REGISTRATION STATEMENT AND COMMON EQUITY OFFERINGS</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>15</Position>
    </Report>
    <Report instance="gain-20230630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R16.htm</HtmlFileName>
      <LongName>0000016 - Disclosure - NET INCREASE (DECREASE) IN NET ASSETS RESULTING FROM OPERATIONS PER WEIGHTED-AVERAGE COMMON SHARE</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.gladstoneinvestment.com/role/NETINCREASEDECREASEINNETASSETSRESULTINGFROMOPERATIONSPERWEIGHTEDAVERAGECOMMONSHARE</Role>
      <ShortName>NET INCREASE (DECREASE) IN NET ASSETS RESULTING FROM OPERATIONS PER WEIGHTED-AVERAGE COMMON SHARE</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>16</Position>
    </Report>
    <Report instance="gain-20230630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R17.htm</HtmlFileName>
      <LongName>0000017 - Disclosure - DISTRIBUTIONS TO COMMON STOCKHOLDERS</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.gladstoneinvestment.com/role/DISTRIBUTIONSTOCOMMONSTOCKHOLDERS</Role>
      <ShortName>DISTRIBUTIONS TO COMMON STOCKHOLDERS</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>17</Position>
    </Report>
    <Report instance="gain-20230630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R18.htm</HtmlFileName>
      <LongName>0000018 - Disclosure - COMMITMENTS AND CONTINGENCIES</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.gladstoneinvestment.com/role/COMMITMENTSANDCONTINGENCIES</Role>
      <ShortName>COMMITMENTS AND CONTINGENCIES</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>18</Position>
    </Report>
    <Report instance="gain-20230630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R19.htm</HtmlFileName>
      <LongName>0000019 - Disclosure - FINANCIAL HIGHLIGHTS</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.gladstoneinvestment.com/role/FINANCIALHIGHLIGHTS</Role>
      <ShortName>FINANCIAL HIGHLIGHTS</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>19</Position>
    </Report>
    <Report instance="gain-20230630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R20.htm</HtmlFileName>
      <LongName>0000020 - Disclosure - UNCONSOLIDATED SIGNIFICANT SUBSIDIARIES</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.gladstoneinvestment.com/role/UNCONSOLIDATEDSIGNIFICANTSUBSIDIARIES</Role>
      <ShortName>UNCONSOLIDATED SIGNIFICANT SUBSIDIARIES</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>20</Position>
    </Report>
    <Report instance="gain-20230630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R21.htm</HtmlFileName>
      <LongName>0000021 - Disclosure - SUBSEQUENT EVENTS</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.gladstoneinvestment.com/role/SUBSEQUENTEVENTS</Role>
      <ShortName>SUBSEQUENT EVENTS</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>21</Position>
    </Report>
    <Report instance="gain-20230630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R22.htm</HtmlFileName>
      <LongName>995410 - Disclosure - Pay vs Performance Disclosure</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://xbrl.sec.gov/ecd/role/PvpDisclosure</Role>
      <ShortName>Pay vs Performance Disclosure</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>22</Position>
    </Report>
    <Report instance="gain-20230630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R23.htm</HtmlFileName>
      <LongName>995445 - Disclosure - Insider Trading Arrangements</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://xbrl.sec.gov/ecd/role/InsiderTradingArrangements</Role>
      <ShortName>Insider Trading Arrangements</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>23</Position>
    </Report>
    <Report instance="gain-20230630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R24.htm</HtmlFileName>
      <LongName>995470 - Disclosure - N-2</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://xbrl.sec.gov/cef/role/N2</Role>
      <ShortName>N-2</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>24</Position>
    </Report>
    <Report instance="gain-20230630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R25.htm</HtmlFileName>
      <LongName>9954701 - Disclosure - SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES (Policies)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.gladstoneinvestment.com/role/SUMMARYOFSIGNIFICANTACCOUNTINGPOLICIESPolicies</Role>
      <ShortName>SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES (Policies)</ShortName>
      <MenuCategory>Policies</MenuCategory>
      <Position>25</Position>
    </Report>
    <Report instance="gain-20230630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R26.htm</HtmlFileName>
      <LongName>9954702 - Disclosure - INVESTMENTS (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.gladstoneinvestment.com/role/INVESTMENTSTables</Role>
      <ShortName>INVESTMENTS (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.gladstoneinvestment.com/role/INVESTMENTS</ParentRole>
      <Position>26</Position>
    </Report>
    <Report instance="gain-20230630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R27.htm</HtmlFileName>
      <LongName>9954703 - Disclosure - RELATED PARTY TRANSACTIONS (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.gladstoneinvestment.com/role/RELATEDPARTYTRANSACTIONSTables</Role>
      <ShortName>RELATED PARTY TRANSACTIONS (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.gladstoneinvestment.com/role/RELATEDPARTYTRANSACTIONS</ParentRole>
      <Position>27</Position>
    </Report>
    <Report instance="gain-20230630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R28.htm</HtmlFileName>
      <LongName>9954704 - Disclosure - BORROWINGS (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.gladstoneinvestment.com/role/BORROWINGSTables</Role>
      <ShortName>BORROWINGS (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.gladstoneinvestment.com/role/BORROWINGS</ParentRole>
      <Position>28</Position>
    </Report>
    <Report instance="gain-20230630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R29.htm</HtmlFileName>
      <LongName>9954705 - Disclosure - NET INCREASE (DECREASE) IN NET ASSETS RESULTING FROM OPERATIONS PER WEIGHTED-AVERAGE COMMON SHARE (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.gladstoneinvestment.com/role/NETINCREASEDECREASEINNETASSETSRESULTINGFROMOPERATIONSPERWEIGHTEDAVERAGECOMMONSHARETables</Role>
      <ShortName>NET INCREASE (DECREASE) IN NET ASSETS RESULTING FROM OPERATIONS PER WEIGHTED-AVERAGE COMMON SHARE (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.gladstoneinvestment.com/role/NETINCREASEDECREASEINNETASSETSRESULTINGFROMOPERATIONSPERWEIGHTEDAVERAGECOMMONSHARE</ParentRole>
      <Position>29</Position>
    </Report>
    <Report instance="gain-20230630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R30.htm</HtmlFileName>
      <LongName>9954706 - Disclosure - DISTRIBUTIONS TO COMMON STOCKHOLDERS (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.gladstoneinvestment.com/role/DISTRIBUTIONSTOCOMMONSTOCKHOLDERSTables</Role>
      <ShortName>DISTRIBUTIONS TO COMMON STOCKHOLDERS (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.gladstoneinvestment.com/role/DISTRIBUTIONSTOCOMMONSTOCKHOLDERS</ParentRole>
      <Position>30</Position>
    </Report>
    <Report instance="gain-20230630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R31.htm</HtmlFileName>
      <LongName>9954707 - Disclosure - COMMITMENTS AND CONTINGENCIES (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.gladstoneinvestment.com/role/COMMITMENTSANDCONTINGENCIESTables</Role>
      <ShortName>COMMITMENTS AND CONTINGENCIES (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.gladstoneinvestment.com/role/COMMITMENTSANDCONTINGENCIES</ParentRole>
      <Position>31</Position>
    </Report>
    <Report instance="gain-20230630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R32.htm</HtmlFileName>
      <LongName>9954708 - Disclosure - FINANCIAL HIGHLIGHTS (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.gladstoneinvestment.com/role/FINANCIALHIGHLIGHTSTables</Role>
      <ShortName>FINANCIAL HIGHLIGHTS (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.gladstoneinvestment.com/role/FINANCIALHIGHLIGHTS</ParentRole>
      <Position>32</Position>
    </Report>
    <Report instance="gain-20230630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R33.htm</HtmlFileName>
      <LongName>9954709 - Disclosure - SUBSEQUENT EVENTS (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.gladstoneinvestment.com/role/SUBSEQUENTEVENTSTables</Role>
      <ShortName>SUBSEQUENT EVENTS (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.gladstoneinvestment.com/role/SUBSEQUENTEVENTS</ParentRole>
      <Position>33</Position>
    </Report>
    <Report instance="gain-20230630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R34.htm</HtmlFileName>
      <LongName>9954710 - Disclosure - ORGANIZATION (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.gladstoneinvestment.com/role/ORGANIZATIONDetails</Role>
      <ShortName>ORGANIZATION (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.gladstoneinvestment.com/role/ORGANIZATION</ParentRole>
      <Position>34</Position>
    </Report>
    <Report instance="gain-20230630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R35.htm</HtmlFileName>
      <LongName>9954711 - Disclosure - SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.gladstoneinvestment.com/role/SUMMARYOFSIGNIFICANTACCOUNTINGPOLICIESDetails</Role>
      <ShortName>SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.gladstoneinvestment.com/role/SUMMARYOFSIGNIFICANTACCOUNTINGPOLICIESPolicies</ParentRole>
      <Position>35</Position>
    </Report>
    <Report instance="gain-20230630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R36.htm</HtmlFileName>
      <LongName>9954712 - Disclosure - INVESTMENTS - Fair Value, Assets Measured on Recurring Basis (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.gladstoneinvestment.com/role/INVESTMENTSFairValueAssetsMeasuredonRecurringBasisDetails</Role>
      <ShortName>INVESTMENTS - Fair Value, Assets Measured on Recurring Basis (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>36</Position>
    </Report>
    <Report instance="gain-20230630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R37.htm</HtmlFileName>
      <LongName>9954713 - Disclosure - INVESTMENTS - Fair Value Measurement Inputs and Valuation Techniques (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.gladstoneinvestment.com/role/INVESTMENTSFairValueMeasurementInputsandValuationTechniquesDetails</Role>
      <ShortName>INVESTMENTS - Fair Value Measurement Inputs and Valuation Techniques (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>37</Position>
    </Report>
    <Report instance="gain-20230630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R38.htm</HtmlFileName>
      <LongName>9954714 - Disclosure - INVESTMENTS - Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.gladstoneinvestment.com/role/INVESTMENTSFairValueAssetsMeasuredonRecurringBasisUnobservableInputReconciliationDetails</Role>
      <ShortName>INVESTMENTS - Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>38</Position>
    </Report>
    <Report instance="gain-20230630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R39.htm</HtmlFileName>
      <LongName>9954715 - Disclosure - INVESTMENTS - Narrative (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.gladstoneinvestment.com/role/INVESTMENTSNarrativeDetails</Role>
      <ShortName>INVESTMENTS - Narrative (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>39</Position>
    </Report>
    <Report instance="gain-20230630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R40.htm</HtmlFileName>
      <LongName>9954716 - Disclosure - INVESTMENTS - Summary Investment Holdings (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.gladstoneinvestment.com/role/INVESTMENTSSummaryInvestmentHoldingsDetails</Role>
      <ShortName>INVESTMENTS - Summary Investment Holdings (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>40</Position>
    </Report>
    <Report instance="gain-20230630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R41.htm</HtmlFileName>
      <LongName>9954717 - Disclosure - INVESTMENTS - Investments Classified by Contractual Maturity Date (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.gladstoneinvestment.com/role/INVESTMENTSInvestmentsClassifiedbyContractualMaturityDateDetails</Role>
      <ShortName>INVESTMENTS - Investments Classified by Contractual Maturity Date (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>41</Position>
    </Report>
    <Report instance="gain-20230630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R42.htm</HtmlFileName>
      <LongName>9954718 - Disclosure - RELATED PARTY TRANSACTIONS - Narrative (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.gladstoneinvestment.com/role/RELATEDPARTYTRANSACTIONSNarrativeDetails</Role>
      <ShortName>RELATED PARTY TRANSACTIONS - Narrative (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>42</Position>
    </Report>
    <Report instance="gain-20230630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R43.htm</HtmlFileName>
      <LongName>9954719 - Disclosure - RELATED PARTY TRANSACTIONS - Schedule of Related Party Transactions (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.gladstoneinvestment.com/role/RELATEDPARTYTRANSACTIONSScheduleofRelatedPartyTransactionsDetails</Role>
      <ShortName>RELATED PARTY TRANSACTIONS - Schedule of Related Party Transactions (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>43</Position>
    </Report>
    <Report instance="gain-20230630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R44.htm</HtmlFileName>
      <LongName>9954720 - Disclosure - RELATED PARTY TRANSACTIONS - Schedule of Related Party Fees Due (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.gladstoneinvestment.com/role/RELATEDPARTYTRANSACTIONSScheduleofRelatedPartyFeesDueDetails</Role>
      <ShortName>RELATED PARTY TRANSACTIONS - Schedule of Related Party Fees Due (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>44</Position>
    </Report>
    <Report instance="gain-20230630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R45.htm</HtmlFileName>
      <LongName>9954721 - Disclosure - BORROWINGS - Revolving Line of Credit Narrative (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.gladstoneinvestment.com/role/BORROWINGSRevolvingLineofCreditNarrativeDetails</Role>
      <ShortName>BORROWINGS - Revolving Line of Credit Narrative (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>45</Position>
    </Report>
    <Report instance="gain-20230630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R46.htm</HtmlFileName>
      <LongName>9954722 - Disclosure - BORROWINGS - Schedule of Line of Credit Facilities (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.gladstoneinvestment.com/role/BORROWINGSScheduleofLineofCreditFacilitiesDetails</Role>
      <ShortName>BORROWINGS - Schedule of Line of Credit Facilities (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>46</Position>
    </Report>
    <Report instance="gain-20230630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R47.htm</HtmlFileName>
      <LongName>9954723 - Disclosure - BORROWINGS - Fair Value Narrative (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.gladstoneinvestment.com/role/BORROWINGSFairValueNarrativeDetails</Role>
      <ShortName>BORROWINGS - Fair Value Narrative (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>47</Position>
    </Report>
    <Report instance="gain-20230630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R48.htm</HtmlFileName>
      <LongName>9954724 - Disclosure - BORROWINGS - Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.gladstoneinvestment.com/role/BORROWINGSFairValueLiabilitiesMeasuredonRecurringBasisUnobservableInputReconciliationDetails</Role>
      <ShortName>BORROWINGS - Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>48</Position>
    </Report>
    <Report instance="gain-20230630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R49.htm</HtmlFileName>
      <LongName>9954725 - Disclosure - BORROWINGS - Notes Payable Narrative (Details)</LongName>
      <ReportType>Notes</ReportType>
      <Role>http://www.gladstoneinvestment.com/role/BORROWINGSNotesPayableNarrativeDetails</Role>
      <ShortName>BORROWINGS - Notes Payable Narrative (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>49</Position>
    </Report>
    <Report instance="gain-20230630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R50.htm</HtmlFileName>
      <LongName>9954726 - Disclosure - BORROWINGS - Schedule of Debt (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.gladstoneinvestment.com/role/BORROWINGSScheduleofDebtDetails</Role>
      <ShortName>BORROWINGS - Schedule of Debt (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>50</Position>
    </Report>
    <Report instance="gain-20230630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R51.htm</HtmlFileName>
      <LongName>9954727 - Disclosure - REGISTRATION STATEMENT AND COMMON EQUITY OFFERINGS (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.gladstoneinvestment.com/role/REGISTRATIONSTATEMENTANDCOMMONEQUITYOFFERINGSDetails</Role>
      <ShortName>REGISTRATION STATEMENT AND COMMON EQUITY OFFERINGS (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.gladstoneinvestment.com/role/REGISTRATIONSTATEMENTANDCOMMONEQUITYOFFERINGS</ParentRole>
      <Position>51</Position>
    </Report>
    <Report instance="gain-20230630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R52.htm</HtmlFileName>
      <LongName>9954728 - Disclosure - NET INCREASE (DECREASE) IN NET ASSETS RESULTING FROM OPERATIONS PER WEIGHTED-AVERAGE COMMON SHARE (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.gladstoneinvestment.com/role/NETINCREASEDECREASEINNETASSETSRESULTINGFROMOPERATIONSPERWEIGHTEDAVERAGECOMMONSHAREDetails</Role>
      <ShortName>NET INCREASE (DECREASE) IN NET ASSETS RESULTING FROM OPERATIONS PER WEIGHTED-AVERAGE COMMON SHARE (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.gladstoneinvestment.com/role/NETINCREASEDECREASEINNETASSETSRESULTINGFROMOPERATIONSPERWEIGHTEDAVERAGECOMMONSHARETables</ParentRole>
      <Position>52</Position>
    </Report>
    <Report instance="gain-20230630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R53.htm</HtmlFileName>
      <LongName>9954729 - Disclosure - DISTRIBUTIONS TO COMMON STOCKHOLDERS - Narrative (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.gladstoneinvestment.com/role/DISTRIBUTIONSTOCOMMONSTOCKHOLDERSNarrativeDetails</Role>
      <ShortName>DISTRIBUTIONS TO COMMON STOCKHOLDERS - Narrative (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>53</Position>
    </Report>
    <Report instance="gain-20230630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R54.htm</HtmlFileName>
      <LongName>9954730 - Disclosure - DISTRIBUTIONS TO COMMON STOCKHOLDERS - Dividends Declared (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.gladstoneinvestment.com/role/DISTRIBUTIONSTOCOMMONSTOCKHOLDERSDividendsDeclaredDetails</Role>
      <ShortName>DISTRIBUTIONS TO COMMON STOCKHOLDERS - Dividends Declared (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>54</Position>
    </Report>
    <Report instance="gain-20230630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R55.htm</HtmlFileName>
      <LongName>9954731 - Disclosure - COMMITMENTS AND CONTINGENCIES - Narrative (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.gladstoneinvestment.com/role/COMMITMENTSANDCONTINGENCIESNarrativeDetails</Role>
      <ShortName>COMMITMENTS AND CONTINGENCIES - Narrative (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>55</Position>
    </Report>
    <Report instance="gain-20230630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R56.htm</HtmlFileName>
      <LongName>9954732 - Disclosure - COMMITMENTS AND CONTINGENCIES - Schedule of Principal Balances of Unused Line of Credit and Delayed Draw Term Debt Commitments and Guaranties (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.gladstoneinvestment.com/role/COMMITMENTSANDCONTINGENCIESScheduleofPrincipalBalancesofUnusedLineofCreditandDelayedDrawTermDebtCommitmentsandGuarantiesDetails</Role>
      <ShortName>COMMITMENTS AND CONTINGENCIES - Schedule of Principal Balances of Unused Line of Credit and Delayed Draw Term Debt Commitments and Guaranties (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>56</Position>
    </Report>
    <Report instance="gain-20230630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R57.htm</HtmlFileName>
      <LongName>9954733 - Disclosure - FINANCIAL HIGHLIGHTS (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.gladstoneinvestment.com/role/FINANCIALHIGHLIGHTSDetails</Role>
      <ShortName>FINANCIAL HIGHLIGHTS (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.gladstoneinvestment.com/role/FINANCIALHIGHLIGHTSTables</ParentRole>
      <Position>57</Position>
    </Report>
    <Report instance="gain-20230630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R58.htm</HtmlFileName>
      <LongName>9954734 - Disclosure - SUBSEQUENT EVENTS - Schedule of Subsequent Events (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.gladstoneinvestment.com/role/SUBSEQUENTEVENTSScheduleofSubsequentEventsDetails</Role>
      <ShortName>SUBSEQUENT EVENTS - Schedule of Subsequent Events (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>58</Position>
    </Report>
    <Report instance="gain-20230630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R59.htm</HtmlFileName>
      <LongName>9954735 - Disclosure - SUBSEQUENT EVENTS-Narratives (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.gladstoneinvestment.com/role/SUBSEQUENTEVENTSNarrativesDetails</Role>
      <ShortName>SUBSEQUENT EVENTS-Narratives (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>59</Position>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <LongName>All Reports</LongName>
      <ReportType>Book</ReportType>
      <ShortName>All Reports</ShortName>
    </Report>
  </MyReports>
  <Logs>
    <Log type="Warning">[ix-0514-Hidden-Fact-Eligible-For-Transform] WARN: 1 fact(s) appearing in ix:hidden were eligible for transformation: dei:EntityRegistrantName -  gain-20230630.htm 4</Log>
  </Logs>
  <InputFiles>
    <File doctype="10-Q" original="gain-20230630.htm">gain-20230630.htm</File>
    <File>gain-20230630.xsd</File>
    <File>gain-20230630_cal.xml</File>
    <File>gain-20230630_def.xml</File>
    <File>gain-20230630_lab.xml</File>
    <File>gain-20230630_pre.xml</File>
    <File>gain63023fy24q110-qex311.htm</File>
    <File>gain63023fy24q110-qex312.htm</File>
    <File>gain63023fy24q110-qex321.htm</File>
    <File>gain63023fy24q110-qex322.htm</File>
  </InputFiles>
  <SupplementalFiles/>
  <BaseTaxonomies>
    <BaseTaxonomy items="1632">http://fasb.org/us-gaap/2023</BaseTaxonomy>
    <BaseTaxonomy items="19">http://xbrl.sec.gov/cef/2023</BaseTaxonomy>
    <BaseTaxonomy items="39">http://xbrl.sec.gov/dei/2023</BaseTaxonomy>
    <BaseTaxonomy items="4">http://xbrl.sec.gov/ecd/2023</BaseTaxonomy>
  </BaseTaxonomies>
  <HasPresentationLinkbase>true</HasPresentationLinkbase>
  <HasCalculationLinkbase>true</HasCalculationLinkbase>
</FilingSummary>
