Quarterly report pursuant to Section 13 or 15(d)

RELATED PARTY TRANSACTIONS - Narrative (Details)

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RELATED PARTY TRANSACTIONS - Narrative (Details) - USD ($)
 3 Months Ended 6 Months Ended 12 Months Ended 36 Months Ended
Mar. 31, 2024
Mar. 31, 2023
Mar. 31, 2024
Mar. 31, 2023
Mar. 31, 2023
Mar. 31, 2023
Sep. 30, 2023
Related Party Transaction [Line Items]              
Base management fee 1.75% 1.75% 1.75% 1.75%      
Base management fee, percent fee     100.00%        
Management and service fee, net of credit 0.50% 0.50% 0.50% 0.50%      
Management and service fees, loans     1.50%        
Ratio of net investment income (loss) to average net assets – annualized 7.62% 8.30% 7.80% 8.09%      
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%   2.50%    
Pre-incentive fee net investment income exceeds the hurdle rate     20.00%        
Investment company, capital gains-based incentive fees 20.00%   20.00%        
Success fee income $ 0 $ 600,000 $ 0 $ 600,000      
Investment company, other operating expenses, due to advisor $ 200,000   $ 200,000       $ 65,000
Gladstone Securities              
Related Party Transaction [Line Items]              
Selling commissions, percentage 7.00%   7.00%        
Dealer manager fees percentage 3.00%   3.00%        
Payments for selling commissions and dealer-manager fees $ 200,000 0 $ 200,000 0      
Related Party              
Related Party Transaction [Line Items]              
Success fee income 100,000 $ 200,000 100,000 $ 300,000      
Fee due to related party 4,611,000   4,611,000       4,351,000
Related Party | Gladstone Investment Corporation              
Related Party Transaction [Line Items]              
Fee due to related party $ 66,000   $ 66,000       $ 19,000
Adviser | Chief Executive Officer | Related Party              
Related Party Transaction [Line Items]              
Noncontrolling interest, ownership percentage by parent 100.00%   100.00%        
Administrator | Chief Executive Officer | Related Party              
Related Party Transaction [Line Items]              
Noncontrolling interest, ownership percentage by parent 100.00%   100.00%        
Gladstone Securities, LLC | Chief Executive Officer | Related Party              
Related Party Transaction [Line Items]              
Noncontrolling interest, ownership percentage by parent 100.00%   100.00%        
Maximum              
Related Party Transaction [Line Items]              
Base management fee     1.75%        
Minimum              
Related Party Transaction [Line Items]              
Ratio of net investment income (loss) to average net assets – annualized     1.75%     2.00%