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INVESTMENTS - Schedule of Fair Value Measurement Inputs and Valuation Techniques (Details) $ in Thousands | Sep. 30, 2025 USD ($) investment | Sep. 30, 2024 USD ($)
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Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |  |  |
Investments owned, fair value | $ 859,124 | [1] | $ 796,260 | [2]
Investments at fair value | 859,124 |  | 796,260 |
Gladstone Alternative Income Fund |  |  |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |  |  |
Investments at fair value | 5,100 |  |  |
Leeds Acquisition Holdings, LLC |  |  |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |  |  |
Investments at fair value | 36 |  | 38 |
Secured first lien debt |  |  |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |  |  |
Investments at fair value | 622,371 |  | 554,937 |
Secured second lien debt |  |  |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |  |  |
Investments at fair value | 150,542 |  | 113,716 |
Unsecured debt |  |  |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |  |  |
Investments at fair value | 333 |  | 32 |
Significant Unobservable Inputs (Level 3) |  |  |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |  |  |
Investments owned, fair value | 854,013 |  | 796,222 |
Investments at fair value | 854,013 |  | 796,222 |
Significant Unobservable Inputs (Level 3) | Secured first lien debt |  |  |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |  |  |
Investments at fair value | 622,371 |  | 554,937 |
Significant Unobservable Inputs (Level 3) | Secured second lien debt |  |  |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |  |  |
Investments at fair value | 150,542 |  | 113,716 |
Significant Unobservable Inputs (Level 3) | Unsecured debt |  |  |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |  |  |
Investments at fair value | 333 |  | 32 |
Significant Unobservable Inputs (Level 3) | Expected Payoff Amount | Preferred and common equity / equivalents |  |  |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |  |  |
Investments owned, fair value | $ 2,200 |  |  |
Investment owned, number of investments | investment | 1 |  |  |
Significant Unobservable Inputs (Level 3) | Yield Analysis | Secured first lien debt |  |  |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |  |  |
Investments owned, fair value | $ 548,670 |  | 464,090 |
Significant Unobservable Inputs (Level 3) | Yield Analysis | Secured second lien debt |  |  |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |  |  |
Investments owned, fair value | $ 141,750 |  | $ 101,928 |
Significant Unobservable Inputs (Level 3) | Yield Analysis | Discount Rate | Secured first lien debt | Minimum |  |  |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |  |  |
Investment owned, measurement input | 0.094 |  | 0.108 |
Significant Unobservable Inputs (Level 3) | Yield Analysis | Discount Rate | Secured first lien debt | Maximum |  |  |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |  |  |
Investment owned, measurement input | 0.198 |  | 0.173 |
Significant Unobservable Inputs (Level 3) | Yield Analysis | Discount Rate | Secured first lien debt | Weighted Average |  |  |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |  |  |
Investment owned, measurement input | 0.117 |  | 0.126 |
Significant Unobservable Inputs (Level 3) | Yield Analysis | Discount Rate | Secured second lien debt | Minimum |  |  |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |  |  |
Investment owned, measurement input | 0.114 |  | 0.122 |
Significant Unobservable Inputs (Level 3) | Yield Analysis | Discount Rate | Secured second lien debt | Maximum |  |  |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |  |  |
Investment owned, measurement input | 0.152 |  | 0.160 |
Significant Unobservable Inputs (Level 3) | Yield Analysis | Discount Rate | Secured second lien debt | Weighted Average |  |  |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |  |  |
Investment owned, measurement input | 0.131 |  | 0.141 |
Significant Unobservable Inputs (Level 3) | TEV | Secured first lien debt |  |  |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |  |  |
Investments owned, fair value | $ 73,701 |  | $ 90,847 |
Significant Unobservable Inputs (Level 3) | TEV | Secured second lien debt |  |  |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |  |  |
Investments owned, fair value | 8,792 |  | 8,547 |
Significant Unobservable Inputs (Level 3) | TEV | Unsecured debt |  |  |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |  |  |
Investments owned, fair value | 333 |  | 32 |
Significant Unobservable Inputs (Level 3) | TEV | Preferred and common equity / equivalents |  |  |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |  |  |
Investments owned, fair value | $ 80,767 |  | $ 127,537 |
Significant Unobservable Inputs (Level 3) | TEV | EBITDA multiple | Secured first lien debt | Minimum |  |  |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |  |  |
Investment owned, measurement input | 3.5 |  | 4.1 |
Significant Unobservable Inputs (Level 3) | TEV | EBITDA multiple | Secured first lien debt | Maximum |  |  |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |  |  |
Investment owned, measurement input | 8.0 |  | 13.9 |
Significant Unobservable Inputs (Level 3) | TEV | EBITDA multiple | Secured first lien debt | Weighted Average |  |  |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |  |  |
Investment owned, measurement input | 6.0 |  | 10.0 |
Significant Unobservable Inputs (Level 3) | TEV | EBITDA multiple | Secured second lien debt | Minimum |  |  |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |  |  |
Investment owned, measurement input | 5.4 |  | 5.4 |
Significant Unobservable Inputs (Level 3) | TEV | EBITDA multiple | Secured second lien debt | Maximum |  |  |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |  |  |
Investment owned, measurement input | 5.4 |  | 5.4 |
Significant Unobservable Inputs (Level 3) | TEV | EBITDA multiple | Secured second lien debt | Weighted Average |  |  |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |  |  |
Investment owned, measurement input | 5.4 |  | 5.4 |
Significant Unobservable Inputs (Level 3) | TEV | EBITDA multiple | Unsecured debt | Minimum |  |  |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |  |  |
Investment owned, measurement input | 8.0 |  | 0.0 |
Significant Unobservable Inputs (Level 3) | TEV | EBITDA multiple | Unsecured debt | Maximum |  |  |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |  |  |
Investment owned, measurement input | 8.0 |  | 0.0 |
Significant Unobservable Inputs (Level 3) | TEV | EBITDA multiple | Unsecured debt | Weighted Average |  |  |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |  |  |
Investment owned, measurement input | 8.0 |  | 0.0 |
Significant Unobservable Inputs (Level 3) | TEV | EBITDA multiple | Preferred and common equity / equivalents | Minimum |  |  |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |  |  |
Investment owned, measurement input | 3.5 |  | 4.1 |
Significant Unobservable Inputs (Level 3) | TEV | EBITDA multiple | Preferred and common equity / equivalents | Maximum |  |  |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |  |  |
Investment owned, measurement input | 14.6 |  | 13.9 |
Significant Unobservable Inputs (Level 3) | TEV | EBITDA multiple | Preferred and common equity / equivalents | Weighted Average |  |  |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |  |  |
Investment owned, measurement input | 6.8 |  | 8.0 |
Significant Unobservable Inputs (Level 3) | TEV | EBITDA | Secured first lien debt | Minimum |  |  |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |  |  |
Investments owned, fair value | $ 538 |  | $ 3,020 |
Significant Unobservable Inputs (Level 3) | TEV | EBITDA | Secured first lien debt | Maximum |  |  |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |  |  |
Investments owned, fair value | 4,655 |  | 16,211 |
Significant Unobservable Inputs (Level 3) | TEV | EBITDA | Secured first lien debt | Weighted Average |  |  |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |  |  |
Investments owned, fair value | 3,540 |  | 10,309 |
Significant Unobservable Inputs (Level 3) | TEV | EBITDA | Secured second lien debt | Minimum |  |  |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |  |  |
Investments owned, fair value | 2,345 |  | 3,343 |
Significant Unobservable Inputs (Level 3) | TEV | EBITDA | Secured second lien debt | Maximum |  |  |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |  |  |
Investments owned, fair value | 2,345 |  | 3,343 |
Significant Unobservable Inputs (Level 3) | TEV | EBITDA | Secured second lien debt | Weighted Average |  |  |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |  |  |
Investments owned, fair value | 2,345 |  | 3,343 |
Significant Unobservable Inputs (Level 3) | TEV | EBITDA | Unsecured debt | Minimum |  |  |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |  |  |
Investments owned, fair value | 3,804 |  | 0 |
Significant Unobservable Inputs (Level 3) | TEV | EBITDA | Unsecured debt | Maximum |  |  |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |  |  |
Investments owned, fair value | 3,804 |  | 0 |
Significant Unobservable Inputs (Level 3) | TEV | EBITDA | Unsecured debt | Weighted Average |  |  |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |  |  |
Investments owned, fair value | 3,804 |  | 0 |
Significant Unobservable Inputs (Level 3) | TEV | EBITDA | Preferred and common equity / equivalents | Minimum |  |  |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |  |  |
Investments owned, fair value | 538 |  | 1,182 |
Significant Unobservable Inputs (Level 3) | TEV | EBITDA | Preferred and common equity / equivalents | Maximum |  |  |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |  |  |
Investments owned, fair value | 142,549 |  | 144,458 |
Significant Unobservable Inputs (Level 3) | TEV | EBITDA | Preferred and common equity / equivalents | Weighted Average |  |  |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |  |  |
Investments owned, fair value | $ 10,222 |  | $ 10,847 |
Significant Unobservable Inputs (Level 3) | TEV | Revenue multiple | Secured first lien debt | Minimum |  |  |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |  |  |
Investment owned, measurement input | 0.6 |  | 0.2 |
Significant Unobservable Inputs (Level 3) | TEV | Revenue multiple | Secured first lien debt | Maximum |  |  |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |  |  |
Investment owned, measurement input | 0.8 |  | 4.6 |
Significant Unobservable Inputs (Level 3) | TEV | Revenue multiple | Secured first lien debt | Weighted Average |  |  |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |  |  |
Investment owned, measurement input | 0.7 |  | 2.1 |
Significant Unobservable Inputs (Level 3) | TEV | Revenue multiple | Unsecured debt | Minimum |  |  |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |  |  |
Investment owned, measurement input | 0.9 |  | 1.0 |
Significant Unobservable Inputs (Level 3) | TEV | Revenue multiple | Unsecured debt | Maximum |  |  |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |  |  |
Investment owned, measurement input | 0.9 |  | 1.0 |
Significant Unobservable Inputs (Level 3) | TEV | Revenue multiple | Unsecured debt | Weighted Average |  |  |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |  |  |
Investment owned, measurement input | 0.9 |  | 1.0 |
Significant Unobservable Inputs (Level 3) | TEV | Revenue multiple | Preferred and common equity / equivalents | Minimum |  |  |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |  |  |
Investment owned, measurement input | 0.6 |  | 0.2 |
Significant Unobservable Inputs (Level 3) | TEV | Revenue multiple | Preferred and common equity / equivalents | Maximum |  |  |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |  |  |
Investment owned, measurement input | 0.9 |  | 4.6 |
Significant Unobservable Inputs (Level 3) | TEV | Revenue multiple | Preferred and common equity / equivalents | Weighted Average |  |  |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |  |  |
Investment owned, measurement input | 0.7 |  | 2.0 |
Significant Unobservable Inputs (Level 3) | TEV | Revenue | Secured first lien debt | Minimum |  |  |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |  |  |
Investments owned, fair value | $ 10,844 |  | $ 6,336 |
Significant Unobservable Inputs (Level 3) | TEV | Revenue | Secured first lien debt | Maximum |  |  |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |  |  |
Investments owned, fair value | 21,649 |  | 21,118 |
Significant Unobservable Inputs (Level 3) | TEV | Revenue | Secured first lien debt | Weighted Average |  |  |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |  |  |
Investments owned, fair value | 15,090 |  | 13,981 |
Significant Unobservable Inputs (Level 3) | TEV | Revenue | Unsecured debt | Minimum |  |  |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |  |  |
Investments owned, fair value | 4,846 |  | 7,834 |
Significant Unobservable Inputs (Level 3) | TEV | Revenue | Unsecured debt | Maximum |  |  |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |  |  |
Investments owned, fair value | 4,846 |  | 7,834 |
Significant Unobservable Inputs (Level 3) | TEV | Revenue | Unsecured debt | Weighted Average |  |  |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |  |  |
Investments owned, fair value | 4,846 |  | 7,834 |
Significant Unobservable Inputs (Level 3) | TEV | Revenue | Preferred and common equity / equivalents | Minimum |  |  |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |  |  |
Investments owned, fair value | 4,846 |  | 4,672 |
Significant Unobservable Inputs (Level 3) | TEV | Revenue | Preferred and common equity / equivalents | Maximum |  |  |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |  |  |
Investments owned, fair value | 21,649 |  | 21,118 |
Significant Unobservable Inputs (Level 3) | TEV | Revenue | Preferred and common equity / equivalents | Weighted Average |  |  |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |  |  |
Investments owned, fair value | 12,394 |  | 12,587 |
Significant Unobservable Inputs (Level 3) | Market Quote | Secured second lien debt |  |  |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |  |  |
Investments owned, fair value | $ 0 |  | $ 3,241 |
Significant Unobservable Inputs (Level 3) | Market Quote | IBP | Secured second lien debt | Minimum |  |  |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |  |  |
Investment owned, measurement input |  |  | 0.880 |
Significant Unobservable Inputs (Level 3) | Market Quote | IBP | Secured second lien debt | Maximum |  |  |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |  |  |
Investment owned, measurement input |  |  | 0.880 |
Significant Unobservable Inputs (Level 3) | Market Quote | IBP | Secured second lien debt | Weighted Average |  |  |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |  |  |
Investment owned, measurement input |  |  | 0.880 |
[1] | Cumulative gross unrealized depreciation for federal income tax purposes is $59.7 million; cumulative gross unrealized appreciation for federal income tax purposes is $34.1 million. Cumulative net unrealized depreciation is $25.5 million, based on a tax cost of $884.7 million.
[2] | Cumulative gross unrealized depreciation for federal income tax purposes is $66.1 million; cumulative gross unrealized appreciation for federal income tax purposes is $85.8 million. Cumulative net unrealized appreciation is $19.7 million, based on a tax cost of $776.5 million

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