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INVESTMENTS - Receivables from Portfolio Companies Narrative (Details) - USD ($) $ in Thousands | 12 Months Ended |
--- | --- | ---
Sep. 30, 2025 | Sep. 30, 2024
Investments, Debt and Equity Securities [Abstract] |  |
Uncollectible receivables allowance, minimum required day for uncollectible adjustment | 90 days |
Gross receivables from portfolio companies | $ 2,400 | $ 1,700
Allowance for uncollectible receivables | $ 52 | $ 75

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- Definition Investment Company, Allowance For Uncollectible Receivables + References No definition available. + Details
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Data Type: | xbrli:monetaryItemType
Balance Type: | credit
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- Definition Investment Company, Gross Receivables From Portfolio Companies + References No definition available. + Details
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Namespace Prefix: | glad_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

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- Definition Uncollectible Receivables Allowance, Minimum Required Day For Uncollectible Adjustment + References No definition available. + Details
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Namespace Prefix: | glad_
Data Type: | xbrli:durationItemType
Balance Type: | na
Period Type: | duration

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- References No definition available. + Details
Name: | us-gaap_InvestmentsDebtAndEquitySecuritiesAbstract
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration