Borrowings - Schedule of Interest Rate and Interest Differential Analysis (Details) - USD ($) $ in Thousands	9 Months Ended
	Jun. 30, 2024	Sep. 30, 2023
Debt Instrument [Line Items]
Amount Outstanding	$ 4,337,697	$ 3,133,332
Class A 2018 Notes | Senior Notes
Debt Instrument [Line Items]
Amount Outstanding	$ 226,794
Class A 2018 Notes | Senior Notes | Secured Overnight Financing Rate (SOFR)
Debt Instrument [Line Items]
Interest Rate	1.48%
Class B 2018 Notes | Senior Notes
Debt Instrument [Line Items]
Amount Outstanding	$ 61,200
Class B 2018 Notes | Senior Notes | Secured Overnight Financing Rate (SOFR)
Debt Instrument [Line Items]
Interest Rate	2.10%
Class C-1 2018 Notes | Senior Notes
Debt Instrument [Line Items]
Amount Outstanding	$ 20,000
Class C-1 2018 Notes | Senior Notes | Secured Overnight Financing Rate (SOFR)
Debt Instrument [Line Items]
Interest Rate	2.80%
2018 Debt Securitization
Debt Instrument [Line Items]
Amount Outstanding	$ 307,994
2018 Debt Securitization | Secured Overnight Financing Rate (SOFR)
Debt Instrument [Line Items]
Interest Rate	0.26161%
Class A-1 GCIC 2018 Notes | Senior Notes
Debt Instrument [Line Items]
Amount Outstanding	$ 323,517
Class A-1 GCIC 2018 Notes | Senior Notes | Secured Overnight Financing Rate (SOFR)
Debt Instrument [Line Items]
Interest Rate	1.48%
Class A-2-R GCIC 2018 Notes | Senior Notes
Debt Instrument [Line Items]
Amount Outstanding	$ 38,500
Interest Rate	2.50%
Class B-1 GCIC 2018 Notes | Senior Notes
Debt Instrument [Line Items]
Amount Outstanding	$ 18,000
Class B-1 GCIC 2018 Notes | Senior Notes | Secured Overnight Financing Rate (SOFR)
Debt Instrument [Line Items]
Interest Rate	2.25%
Class A GBDC 3 2021 Notes | Senior Notes
Debt Instrument [Line Items]
Amount Outstanding	$ 224,000
Class A GBDC 3 2021 Notes | Senior Notes | Secured Overnight Financing Rate (SOFR)
Debt Instrument [Line Items]
Interest Rate	1.60%
Class B GBDC 3 2021 Notes | Senior Notes
Debt Instrument [Line Items]
Amount Outstanding	$ 28,000
Class B GBDC 3 2021 Notes | Senior Notes | Secured Overnight Financing Rate (SOFR)
Debt Instrument [Line Items]
Interest Rate	1.85%
Class C-1 GBDC 3 2021 Notes | Senior Notes
Debt Instrument [Line Items]
Amount Outstanding	$ 36,000
Class C-1 GBDC 3 2021 Notes | Senior Notes | Secured Overnight Financing Rate (SOFR)
Debt Instrument [Line Items]
Interest Rate	2.80%
Class C-2 GBDC 3 2021 Notes | Senior Notes
Debt Instrument [Line Items]
Amount Outstanding	$ 10,000
Interest Rate	3.91%
GBDC 3 2021 Debt Securitization(1)
Debt Instrument [Line Items]
Amount Outstanding	$ 297,950
GBDC 3 2021 Debt Securitization(1) | Secured Overnight Financing Rate (SOFR)
Debt Instrument [Line Items]
Interest Rate	5.30%
GBDC 3 2022 Debt Securitization
Debt Instrument [Line Items]
Amount Outstanding	$ 251,587
GBDC 3 2022 Debt Securitization | Secured Overnight Financing Rate (SOFR)
Debt Instrument [Line Items]
Interest Rate	5.30%
GBDC 3 2022 Debt Securitization | Senior Notes
Debt Instrument [Line Items]
Amount Outstanding	$ 251,587
GBDC 3 2022 Debt Securitization | Senior Notes | Secured Overnight Financing Rate (SOFR)
Debt Instrument [Line Items]
Interest Rate	2.00%
Class A GBDC 3 2022-2 Notes | Senior Notes
Debt Instrument [Line Items]
Amount Outstanding	$ 140,000
Class A GBDC 3 2022-2 Notes | Senior Notes | Secured Overnight Financing Rate (SOFR)
Debt Instrument [Line Items]
Interest Rate	2.60%
Class A GBDC 3 2022-2 Loans | Senior Notes
Debt Instrument [Line Items]
Amount Outstanding	$ 85,000
Class A GBDC 3 2022-2 Loans | Senior Notes | Secured Overnight Financing Rate (SOFR)
Debt Instrument [Line Items]
Interest Rate	2.60%