| Document and Entity Information | Nov. 13, 2023 |
| Document And Entity Information [Line Items] | |
| Entity Registrant Name | GLADSTONE CAPITAL CORP |
| Amendment Flag | false |
| Entity Central Index Key | 0001143513 |
| Document Type | 8-K |
| Document Period End Date | Nov. 13, 2023 |
| Entity Incorporation State Country Code | MD |
| Entity File Number | 814-00237 |
| Entity Tax Identification Number | 54-2040781 |
| Entity Address, Address Line One | 1521 Westbranch Drive |
| Entity Address, Address Line Two | Suite 100 |
| Entity Address, City or Town | McLean |
| Entity Address, State or Province | VA |
| Entity Address, Postal Zip Code | 22102 |
| City Area Code | (703) |
| Local Phone Number | 287-5800 |
| Written Communications | false |
| Soliciting Material | false |
| Pre Commencement Tender Offer | false |
| Pre Commencement Issuer Tender Offer | false |
| Entity Emerging Growth Company | false |
| Common Stock [Member] | |
| Document And Entity Information [Line Items] | |
| Security 12b Title | Common Stock, $0.001 par value per share |
| Trading Symbol | GLAD |
| Security Exchange Name | NASDAQ |
| A 775 Notes Due 2028 [Member] | |
| Document And Entity Information [Line Items] | |
| Security 12b Title | 7.75% notes due 2028 |
| Trading Symbol | GLADZ |
| Security Exchange Name | NASDAQ |
