INVESTMENTS - Schedule of Fair Value, Assets Measured on Recurring Basis (Details) - USD ($) $ in Thousands	Sep. 30, 2025	Sep. 30, 2024
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments at fair value	$ 859,124	$ 796,260
Cash Equivalents	31,774	2,157
Total investments and cash equivalents, fair value	890,898	798,417
Gladstone Alternative Income Fund
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments at fair value	5,100
Leeds Acquisition Holdings, LLC
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments at fair value	36	38
Fair Value, Inputs, Level 1, Level 2, and Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments at fair value	854,013	796,222
Quoted Prices in Active Markets for Identical Assets (Level 1)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments at fair value	0	0
Cash Equivalents	31,774	2,157
Total investments and cash equivalents, fair value	31,774	2,157
Significant Other Observable Inputs (Level 2)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments at fair value	0	0
Cash Equivalents	0	0
Total investments and cash equivalents, fair value	0	0
Significant Unobservable Inputs (Level 3)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments at fair value	854,013	796,222
Cash Equivalents	0	0
Total investments and cash equivalents, fair value	854,013	796,222
Significant Unobservable Inputs (Level 3) \| Fair Value, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments at fair value	854,013	796,222
Significant Unobservable Inputs (Level 3) \| Non-Control/Non-Affiliate Investments \| Fair Value, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments at fair value	696,281	750,866
Significant Unobservable Inputs (Level 3) \| Affiliate Investments \| Fair Value, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments at fair value	48,836	7,438
Significant Unobservable Inputs (Level 3) \| Control Investments \| Fair Value, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments at fair value	108,896	37,918
Investments measured at NAV
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments at fair value	5,111	38
Secured first lien debt
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments at fair value	622,371	554,937
Secured first lien debt \| Quoted Prices in Active Markets for Identical Assets (Level 1)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments at fair value	0	0
Secured first lien debt \| Significant Other Observable Inputs (Level 2)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments at fair value	0	0
Secured first lien debt \| Significant Unobservable Inputs (Level 3)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments at fair value	622,371	554,937
Secured first lien debt \| Significant Unobservable Inputs (Level 3) \| Non-Control/Non-Affiliate Investments \| Fair Value, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments at fair value	517,762	540,661
Secured first lien debt \| Significant Unobservable Inputs (Level 3) \| Affiliate Investments \| Fair Value, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments at fair value	34,713	380
Secured first lien debt \| Significant Unobservable Inputs (Level 3) \| Control Investments \| Fair Value, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments at fair value	69,896	13,896
Secured second lien debt
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments at fair value	150,542	113,716
Secured second lien debt \| Quoted Prices in Active Markets for Identical Assets (Level 1)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments at fair value	0	0
Secured second lien debt \| Significant Other Observable Inputs (Level 2)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments at fair value	0	0
Secured second lien debt \| Significant Unobservable Inputs (Level 3)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments at fair value	150,542	113,716
Secured second lien debt \| Significant Unobservable Inputs (Level 3) \| Non-Control/Non-Affiliate Investments \| Fair Value, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments at fair value	131,750	105,169
Secured second lien debt \| Significant Unobservable Inputs (Level 3) \| Control Investments \| Fair Value, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments at fair value	18,792	8,547
Unsecured debt
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments at fair value	333	32
Unsecured debt \| Quoted Prices in Active Markets for Identical Assets (Level 1)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments at fair value	0	0
Unsecured debt \| Significant Other Observable Inputs (Level 2)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments at fair value	0	0
Unsecured debt \| Significant Unobservable Inputs (Level 3)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments at fair value	333	32
Unsecured debt \| Significant Unobservable Inputs (Level 3) \| Non-Control/Non-Affiliate Investments \| Fair Value, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments at fair value	17	32
Unsecured debt \| Significant Unobservable Inputs (Level 3) \| Control Investments \| Fair Value, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments at fair value	316	0
Preferred equity
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments at fair value	31,214	31,346
Preferred equity \| Quoted Prices in Active Markets for Identical Assets (Level 1)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments at fair value	0	0
Preferred equity \| Significant Other Observable Inputs (Level 2)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments at fair value	0	0
Preferred equity \| Significant Unobservable Inputs (Level 3)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments at fair value	31,214	31,346
Preferred equity \| Significant Unobservable Inputs (Level 3) \| Non-Control/Non-Affiliate Investments \| Fair Value, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments at fair value	16,794	27,247
Preferred equity \| Significant Unobservable Inputs (Level 3) \| Affiliate Investments \| Fair Value, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments at fair value	9,420	4,099
Preferred equity \| Significant Unobservable Inputs (Level 3) \| Control Investments \| Fair Value, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments at fair value	5,000	0
Common equity/equivalents
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments at fair value	49,553	96,191
Common equity/equivalents \| Quoted Prices in Active Markets for Identical Assets (Level 1)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments at fair value	0	0
Common equity/equivalents \| Significant Other Observable Inputs (Level 2)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments at fair value	0	0
Common equity/equivalents \| Significant Unobservable Inputs (Level 3)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments at fair value	49,553	96,191
Common equity/equivalents \| Significant Unobservable Inputs (Level 3) \| Non-Control/Non-Affiliate Investments \| Fair Value, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments at fair value	29,958	77,757
Common equity/equivalents \| Significant Unobservable Inputs (Level 3) \| Affiliate Investments \| Fair Value, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments at fair value	4,703	2,959
Common equity/equivalents \| Significant Unobservable Inputs (Level 3) \| Control Investments \| Fair Value, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments at fair value	$ 14,892	$ 15,475

GLADSTONE CAPITAL CORP 10-K