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RELATED PARTY TRANSACTIONS - Narrative (Details) - USD ($) | 3 Months Ended | 6 Months Ended | 12 Months Ended
--- | --- | --- | ---
Sep. 30, 2025 | Sep. 30, 2024 | Sep. 30, 2025 | Sep. 30, 2024 | Mar. 31, 2025
Related Party Transaction [Line Items] |  |  |  |  |  |
Base management fee (as a percent) |  | 2.00% | 2.00% | 2.00% | 2.00% |
Base management fee, percent fee |  |  |  | 100.00% |  |
Investment company, excess expense reimbursable |  | $ 130,000 | $ 77,000 | $ 239,000 | $ 152,000 |
Maximum base management fee paid cannot exceed a percentage of total assets |  |  |  | 2.00% |  |
Ratio of net investment income (loss) to average net assets – annualized |  |  |  | 1.88% | 5.84% |
Investment company, capital gains-based incentive fees |  |  |  | $ 0 |  | $ 4,900,000
Investment company, capital gains-based incentive fees payable (reversal) |  | 4,900,000 | 1,600,000 | 4,700,000 | $ 2,200,000 |
Accrued capital gains-based incentive fee |  | 44,000,000.0 |  | 44,000,000.0 |  | 39,300,000
Administration fee | [1] | 520,000 | 567,000 | 953,000 | 1,073,000 |
Success fee income |  | 0 | 150,000 | 754,000 | 1,703,000 |
Related Party |  |  |  |  |  |
Related Party Transaction [Line Items] |  |  |  |  |  |
Success fee income |  | 700,000 | $ 200,000 | 1,300,000 | $ 200,000 |
Due from related parties |  | $ 100,000 |  | 100,000 |  | $ 100,000
Affiliated Entity | Advisory Agreement |  |  |  |  |  |
Related Party Transaction [Line Items] |  |  |  |  |  |
Incentive fee base on minimum rate of return required on investment |  |  |  | $ 0 |  |
Percentage of pre-incentive fee net investment income is less than the hurdle rate |  |  |  | 100.00% |  |
Investment company, pre-incentive fee net investment income, percent threshold of net assets |  |  |  | 2.1875% |  |
Pre-incentive fee net investment income exceeds the hurdle rate |  |  |  | 20.00% |  |
Investment company, capital gains-based incentive fees |  | 20.00% |  | 20.00% |  |
Affiliated Entity | Minimum | Advisory Agreement |  |  |  |  |  |
Related Party Transaction [Line Items] |  |  |  |  |  |
Ratio of net investment income (loss) to average net assets – annualized |  |  |  | 1.75% |  |
Adviser | Chief Executive Officer | Related Party |  |  |  |  |  |
Related Party Transaction [Line Items] |  |  |  |  |  |
Ownership percentage by parent |  | 100.00% |  | 100.00% |  |
Administrator | Chief Executive Officer | Related Party |  |  |  |  |  |
Related Party Transaction [Line Items] |  |  |  |  |  |
Ownership percentage by parent |  | 100.00% |  | 100.00% |  |
Gladstone Securities, LLC | Chief Executive Officer | Related Party |  |  |  |  |  |
Related Party Transaction [Line Items] |  |  |  |  |  |
Ownership percentage by parent |  | 100.00% |  | 100.00% |  |
[1] | Refer to Note 4 — Related Party Transactions in the accompanying Notes to Consolidated Financial Statements for additional information.

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