Gladstone Capital Corporation Quarterly report pursuant to Section 13 or 15(d)

Quarterly report pursuant to Section 13 or 15(d)

INVESTMENTS - Narrative (Details)

INVESTMENTS - Narrative (Details)	1 Months Ended				2 Months Ended	3 Months Ended	6 Months Ended		12 Months Ended
INVESTMENTS - Narrative (Details)	Mar. 31, 2023 USD ($) industry portfolioCompany state investment	Jan. 31, 2023 USD ($)	Dec. 31, 2022 USD ($)	Oct. 31, 2022 USD ($)	Nov. 30, 2022 USD ($)	Mar. 31, 2023 USD ($) industry portfolioCompany state investment	Mar. 31, 2023 USD ($) industry portfolioCompany state investment	Mar. 31, 2022 USD ($)	Sep. 30, 2022 USD ($) investment
Summary of Investment Holdings [Line Items]
Number of proprietary investments \| investment	46					46	46		46
Investments owned, fair value	$ 678,759,000					$ 678,759,000	$ 678,759,000		$ 649,615,000
Purchase of investments							77,469,000	$ 121,569,000
Line of credit commitment							58,167,000		74,846,000
Cost	$ 696,031,000					$ 696,031,000	$ 696,031,000		656,053,000
Number of investment portfolio company \| portfolioCompany	51					51	51
Number of states which have invested in the company location \| state	24					24	24
Number of industries that have made investments \| industry	13					13	13
Weighted average investment owned, at cost, amount	$ 13,600,000					$ 13,600,000	$ 13,600,000		12,600,000
Uncollectible receivables allowance, minimum required day for uncollectible adjustment							90 days
Gross receivables from portfolio companies	600,000					600,000	$ 600,000		500,000
Allowance for uncollectible receivables	0					0	0		0
Leeds Novamark Capital I, L.P
Summary of Investment Holdings [Line Items]
Investments owned, fair value	200,000					200,000	$ 200,000
Proceeds from equity					$ 6,000,000
Realized investment gains					$ 4,400,000
R2i Holdings, LLC
Summary of Investment Holdings [Line Items]
Net proceeds from the sale of investments			$ 19,200,000
NeoGraf Solutions, LLC
Summary of Investment Holdings [Line Items]
Purchase of investments		$ 29,000,000
Line of credit commitment		$ 4,500,000
Salt & Straw, LLC
Summary of Investment Holdings [Line Items]
Purchase of investments						$ 6,300,000
Leadpoint Business Services, LLC
Summary of Investment Holdings [Line Items]
Purchase of investments	13,500,000
Line of credit commitment	5,500,000
Funded line of credit	$ 5,500,000
Targus Cayman HoldCo Ltd.
Summary of Investment Holdings [Line Items]
Realized investment gains				$ 5,900,000
Net proceeds from the sale of investments				$ 8,000,000
B. Riley Financial, Inc. – Term Debt
Summary of Investment Holdings [Line Items]
Investment, interest rate, paid in cash	6.75%			6.75%		6.75%	6.75%
Cost				$ 2,400,000
Principal or Proprietary Transactions
Summary of Investment Holdings [Line Items]
Investments owned, fair value	$ 668,500,000					$ 668,500,000	$ 668,500,000		$ 630,800,000
Syndicated Investments
Summary of Investment Holdings [Line Items]
Number of proprietary investments \| investment	5					5	5		6
Investments owned, fair value	$ 10,200,000					$ 10,200,000	$ 10,200,000		$ 18,800,000
Investment Portfolio Benchmark \| Investment Type Concentration Risk \| Principal or Proprietary Transactions
Summary of Investment Holdings [Line Items]
Concentration risk, percentage							98.50%		97.10%
Investment Portfolio Benchmark \| Investment Type Concentration Risk \| Syndicated Investments
Summary of Investment Holdings [Line Items]
Concentration risk, percentage							1.50%		2.90%
Investment Portfolio Benchmark \| Customer Concentration Risk \| Five Largest Portfolio Investments
Summary of Investment Holdings [Line Items]
Investments owned, fair value	$ 178,600,000					$ 178,600,000	$ 178,600,000		$ 174,500,000
Concentration risk, percentage							26.30%		26.90%