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INVESTMENTS - Fair Value Measurement Inputs and Valuation Techniques (Details) $ in Thousands | Mar. 31, 2025 USD ($) | Mar. 31, 2024 USD ($)
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Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |  |  |
Fair Value | $ 979,320 | [1] | $ 920,504 | [2]
Investments at fair value | 979,320 |  | 920,504 |
Secured First Lien Debt |  |  |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |  |  |
Investments at fair value | 514,334 |  | 474,856 |
Secured Second Lien Debt |  |  |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |  |  |
Investments at fair value | 103,580 |  | 138,703 |
Preferred Equity |  |  |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |  |  |
Investments at fair value | 302,163 |  | 213,480 |
Common Equity/ Equivalents |  |  |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |  |  |
Investments at fair value | 54,268 |  | 93,465 |
Common Equity/ Equivalents | Gladstone Alternative Income Fund ("GALT") |  |  |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |  |  |
Investments at fair value | 5,000 |  |  |
Significant Unobservable Inputs (Level 3) |  |  |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |  |  |
Fair Value | 974,345 |  | 920,486 |
Investments at fair value | 974,345 |  | 920,486 |
Significant Unobservable Inputs (Level 3) | Secured First Lien Debt |  |  |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |  |  |
Investments at fair value | 514,334 |  | 474,856 |
Significant Unobservable Inputs (Level 3) | Secured Second Lien Debt |  |  |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |  |  |
Investments at fair value | 103,580 |  | 138,703 |
Significant Unobservable Inputs (Level 3) | Preferred Equity |  |  |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |  |  |
Investments at fair value | 302,163 |  | 213,480 |
Significant Unobservable Inputs (Level 3) | Common Equity/ Equivalents |  |  |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |  |  |
Investments at fair value | 54,268 |  | 93,447 |
Significant Unobservable Inputs (Level 3) | Total Enterprise Value | Secured First Lien Debt |  |  |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |  |  |
Fair Value | 514,334 |  | 474,856 |
Significant Unobservable Inputs (Level 3) | Total Enterprise Value | Secured Second Lien Debt |  |  |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |  |  |
Fair Value | 90,956 |  | 113,703 |
Significant Unobservable Inputs (Level 3) | Total Enterprise Value | Preferred Equity |  |  |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |  |  |
Fair Value | 302,163 |  | 213,480 |
Significant Unobservable Inputs (Level 3) | Total Enterprise Value | Common Equity/ Equivalents |  |  |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |  |  |
Fair Value | $ 54,268 |  | $ 93,447 |
Significant Unobservable Inputs (Level 3) | Total Enterprise Value | EBITDA multiple | Secured First Lien Debt | Minimum |  |  |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |  |  |
Fair value, measurement input | 3.7 |  | 4.2 |
Significant Unobservable Inputs (Level 3) | Total Enterprise Value | EBITDA multiple | Secured First Lien Debt | Maximum |  |  |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |  |  |
Fair value, measurement input | 7.9 |  | 8.8 |
Significant Unobservable Inputs (Level 3) | Total Enterprise Value | EBITDA multiple | Secured First Lien Debt | Weighted Average |  |  |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |  |  |
Fair value, measurement input | 6.0 |  | 6.4 |
Significant Unobservable Inputs (Level 3) | Total Enterprise Value | EBITDA multiple | Secured Second Lien Debt | Minimum |  |  |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |  |  |
Fair value, measurement input | 6.1 |  | 5.1 |
Significant Unobservable Inputs (Level 3) | Total Enterprise Value | EBITDA multiple | Secured Second Lien Debt | Maximum |  |  |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |  |  |
Fair value, measurement input | 7.2 |  | 15.0 |
Significant Unobservable Inputs (Level 3) | Total Enterprise Value | EBITDA multiple | Secured Second Lien Debt | Weighted Average |  |  |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |  |  |
Fair value, measurement input | 6.8 |  | 7.0 |
Significant Unobservable Inputs (Level 3) | Total Enterprise Value | EBITDA multiple | Preferred Equity | Minimum |  |  |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |  |  |
Fair value, measurement input | 3.7 |  | 4.2 |
Significant Unobservable Inputs (Level 3) | Total Enterprise Value | EBITDA multiple | Preferred Equity | Maximum |  |  |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |  |  |
Fair value, measurement input | 7.9 |  | 8.8 |
Significant Unobservable Inputs (Level 3) | Total Enterprise Value | EBITDA multiple | Preferred Equity | Weighted Average |  |  |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |  |  |
Fair value, measurement input | 6.1 |  | 6.1 |
Significant Unobservable Inputs (Level 3) | Total Enterprise Value | EBITDA multiple | Common Equity/ Equivalents | Minimum |  |  |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |  |  |
Fair value, measurement input | 5.5 |  | 5.0 |
Significant Unobservable Inputs (Level 3) | Total Enterprise Value | EBITDA multiple | Common Equity/ Equivalents | Maximum |  |  |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |  |  |
Fair value, measurement input | 7.2 |  | 15.0 |
Significant Unobservable Inputs (Level 3) | Total Enterprise Value | EBITDA multiple | Common Equity/ Equivalents | Weighted Average |  |  |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |  |  |
Fair value, measurement input | 6.8 |  | 6.4 |
Significant Unobservable Inputs (Level 3) | Total Enterprise Value | EBITDA | Secured First Lien Debt | Minimum |  |  |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |  |  |
Fair Value | $ 1,208 |  | $ 1,091 |
Significant Unobservable Inputs (Level 3) | Total Enterprise Value | EBITDA | Secured First Lien Debt | Maximum |  |  |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |  |  |
Fair Value | 25,038 |  | 23,547 |
Significant Unobservable Inputs (Level 3) | Total Enterprise Value | EBITDA | Secured First Lien Debt | Weighted Average |  |  |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |  |  |
Fair Value | 12,162 |  | 10,509 |
Significant Unobservable Inputs (Level 3) | Total Enterprise Value | EBITDA | Secured Second Lien Debt | Minimum |  |  |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |  |  |
Fair Value | 3,637 |  | 5,648 |
Significant Unobservable Inputs (Level 3) | Total Enterprise Value | EBITDA | Secured Second Lien Debt | Maximum |  |  |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |  |  |
Fair Value | 24,234 |  | 23,003 |
Significant Unobservable Inputs (Level 3) | Total Enterprise Value | EBITDA | Secured Second Lien Debt | Weighted Average |  |  |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |  |  |
Fair Value | 16,900 |  | 14,192 |
Significant Unobservable Inputs (Level 3) | Total Enterprise Value | EBITDA | Preferred Equity | Minimum |  |  |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |  |  |
Fair Value | 2,153 |  | 1,091 |
Significant Unobservable Inputs (Level 3) | Total Enterprise Value | EBITDA | Preferred Equity | Maximum |  |  |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |  |  |
Fair Value | 25,038 |  | 23,547 |
Significant Unobservable Inputs (Level 3) | Total Enterprise Value | EBITDA | Preferred Equity | Weighted Average |  |  |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |  |  |
Fair Value | 11,029 |  | 9,502 |
Significant Unobservable Inputs (Level 3) | Total Enterprise Value | EBITDA | Common Equity/ Equivalents | Minimum |  |  |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |  |  |
Fair Value | 1,208 |  | 1,154 |
Significant Unobservable Inputs (Level 3) | Total Enterprise Value | EBITDA | Common Equity/ Equivalents | Maximum |  |  |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |  |  |
Fair Value | 24,234 |  | 63,269 |
Significant Unobservable Inputs (Level 3) | Total Enterprise Value | EBITDA | Common Equity/ Equivalents | Weighted Average |  |  |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |  |  |
Fair Value | $ 18,562 |  | $ 23,615 |
Significant Unobservable Inputs (Level 3) | Total Enterprise Value | Revenue multiple | Secured First Lien Debt | Minimum |  |  |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |  |  |
Fair value, measurement input | 0.3 |  | 0.3 |
Significant Unobservable Inputs (Level 3) | Total Enterprise Value | Revenue multiple | Secured First Lien Debt | Maximum |  |  |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |  |  |
Fair value, measurement input | 0.6 |  | 0.6 |
Significant Unobservable Inputs (Level 3) | Total Enterprise Value | Revenue multiple | Secured First Lien Debt | Weighted Average |  |  |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |  |  |
Fair value, measurement input | 0.4 |  | 0.4 |
Significant Unobservable Inputs (Level 3) | Total Enterprise Value | Revenue multiple | Preferred Equity | Minimum |  |  |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |  |  |
Fair value, measurement input | 0.3 |  | 0.3 |
Significant Unobservable Inputs (Level 3) | Total Enterprise Value | Revenue multiple | Preferred Equity | Maximum |  |  |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |  |  |
Fair value, measurement input | 0.6 |  | 0.6 |
Significant Unobservable Inputs (Level 3) | Total Enterprise Value | Revenue multiple | Preferred Equity | Weighted Average |  |  |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |  |  |
Fair value, measurement input | 0.4 |  | 0.4 |
Significant Unobservable Inputs (Level 3) | Total Enterprise Value | Revenue | Secured First Lien Debt | Minimum |  |  |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |  |  |
Fair Value | $ 6,690 |  | $ 31,586 |
Significant Unobservable Inputs (Level 3) | Total Enterprise Value | Revenue | Secured First Lien Debt | Maximum |  |  |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |  |  |
Fair Value | 102,791 |  | 93,916 |
Significant Unobservable Inputs (Level 3) | Total Enterprise Value | Revenue | Secured First Lien Debt | Weighted Average |  |  |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |  |  |
Fair Value | 72,303 |  | 77,580 |
Significant Unobservable Inputs (Level 3) | Total Enterprise Value | Revenue | Preferred Equity | Minimum |  |  |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |  |  |
Fair Value | 6,690 |  | 31,586 |
Significant Unobservable Inputs (Level 3) | Total Enterprise Value | Revenue | Preferred Equity | Maximum |  |  |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |  |  |
Fair Value | 102,791 |  | 93,916 |
Significant Unobservable Inputs (Level 3) | Total Enterprise Value | Revenue | Preferred Equity | Weighted Average |  |  |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |  |  |
Fair Value | 53,604 |  | 75,099 |
Significant Unobservable Inputs (Level 3) | Yield Analysis | Secured Second Lien Debt |  |  |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |  |  |
Fair Value | $ 12,624 |  | $ 25,000 |
Significant Unobservable Inputs (Level 3) | Yield Analysis | Discount Rate | Secured Second Lien Debt | Minimum |  |  |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |  |  |
Fair value, measurement input | 0.207 |  | 0.138 |
Significant Unobservable Inputs (Level 3) | Yield Analysis | Discount Rate | Secured Second Lien Debt | Maximum |  |  |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |  |  |
Fair value, measurement input | 0.207 |  | 0.138 |
Significant Unobservable Inputs (Level 3) | Yield Analysis | Discount Rate | Secured Second Lien Debt | Weighted Average |  |  |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |  |  |
Fair value, measurement input | 0.207 |  | 0.138 |
Significant Other Observable Inputs (Level 2) |  |  |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |  |  |
Investments at fair value | $ 0 |  | $ 18 |
Significant Other Observable Inputs (Level 2) | Secured First Lien Debt |  |  |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |  |  |
Investments at fair value | 0 |  | 0 |
Significant Other Observable Inputs (Level 2) | Secured Second Lien Debt |  |  |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |  |  |
Investments at fair value | 0 |  | 0 |
Significant Other Observable Inputs (Level 2) | Preferred Equity |  |  |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |  |  |
Investments at fair value | 0 |  | 0 |
Significant Other Observable Inputs (Level 2) | Common Equity/ Equivalents |  |  |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |  |  |
Investments at fair value | $ 0 |  | 18 |
Significant Other Observable Inputs (Level 2) | Common Equity/ Equivalents | Funko Acquisition Holdings, LLC |  |  |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |  |  |
Investments at fair value |  |  | $ 18 |
[1] | Cumulative gross unrealized appreciation for federal income tax purposes is $183.3 million; cumulative gross unrealized depreciation for federal income tax purposes is $144.9 million. Cumulative net unrealized appreciation is $38.5 million, based on a tax cost of $940.9 million.
[2] | Cumulative gross unrealized appreciation for federal income tax purposes is $180.5 million; cumulative gross unrealized depreciation for federal income tax purposes is $115.8 million. Cumulative net unrealized appreciation is $64.7 million, based on a tax cost of $855.8 million.

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