Quarterly report pursuant to Section 13 or 15(d)


Cover - shares
3 Months Ended
Dec. 31, 2022
Feb. 03, 2023
Cover [Abstract]    
Document Type 10-Q  
Document Quarterly Report true  
Document Period End Date Dec. 31, 2022  
Document Transition Report false  
Securities Act File Number 814-00237  
Entity Registrant Name GLADSTONE CAPITAL CORP  
Entity Incorporation, State or Country Code MD  
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  
Title of 12(b) Security Common Stock, $0.001 par value per share  
Trading Symbol GLAD  
Security Exchange Name NASDAQ  
Entity Current Reporting Status Yes  
Entity Interactive Data Current Yes  
Entity Filer Category Non-accelerated Filer  
Entity Small Business false  
Entity Emerging Growth Company false  
Entity Shell Company false  
Entity Common Stock, Shares Outstanding   36,514,223
Entity Central Index Key 0001143513  
Current Fiscal Year End Date --09-30  
Document Fiscal Year Focus 2023  
Document Fiscal Period Focus Q1  
Amendment Flag false