Quarterly report pursuant to Section 13 or 15(d)


Cover - shares
6 Months Ended
Mar. 31, 2024
Apr. 30, 2024
Document Information [Line Items]    
Document Type 10-Q  
Document Quarterly Report true  
Document Period End Date Mar. 31, 2024  
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  
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   21,754,471
Entity Central Index Key 0001143513  
Current Fiscal Year End Date --09-30  
Document Fiscal Year Focus 2024  
Document Fiscal Period Focus Q2  
Amendment Flag false  
Common Stock    
Document Information [Line Items]    
Title of 12(b) Security Common Stock, $0.001 par value per share  
Trading Symbol GLAD  
Security Exchange Name NASDAQ  
7.75% Notes due 2028    
Document Information [Line Items]    
Title of 12(b) Security 7.75% notes due 2028  
Trading Symbol GLADZ  
Security Exchange Name NASDAQ