Our investment objective is to generate current income and capital appreciation by investing primarily in one stop (a loan that combines characteristics of traditional first lien senior secured loans and second lien or subordinated loans and that are often referred to by other middle-market lenders as unitranche loans) and other senior secured loans of U.S. middle-market companies. We also selectively invest in second lien and subordinated loans of, and warrants and minority equity securities in U.S. middle-market companies. We intend to achieve our investment objective by (1) accessing the established loan origination channels developed by Golub Capital, a leading lender to U.S. middle-market companies with over $90.0 billion in capital under management(1) as of April 1, 2026, (2) selecting investments within our core middle-market company focus, (3) partnering with experienced private equity firms, or sponsors, in many cases with whom Golub Capital has invested alongside in the past, (4) implementing the disciplined underwriting standards of Golub Capital and (5) drawing upon the aggregate experience and resources of Golub Capital.

NAV Per Share

$ 15.00

Debt Securitization 2025 [Member]

Capital Stock, Long-Term Debt, and Other Securities [Abstract]

Long Term Debt, Title [Text Block]

2025 Debt Securitization

Long Term Debt, Structuring [Text Block]

2025 Debt Securitization: On June 20, 2025, the Company completed a $1,154,900 term debt securitization (the “2025 Debt Securitization”). The notes offered in the 2025 Debt Securitization (the “2025 Notes”) were issued by the 2025 Issuer and were backed by a diversified portfolio of senior secured loans. The transaction was executed through a private placement which consisted of $667,000 of Class A-1 Senior Secured Floating Rate Notes due 2037, which bear interest at three-month SOFR plus 1.63% (the “Class A-1 2025 Notes”); $40,250 of Class A-2 Senior Secured Floating Rate Notes due 2037, which bear interest at three-month SOFR plus 1.80% (the “Class A-2 2025 Notes”); $92,000 of Class B Senior Secured Floating Rate Notes due 2037, which bear interest at three-month SOFR plus 2.00% (the “Class B 2025 Notes”); $69,000 of Class C Senior Secured Floating Rate Notes due 2037, which bear interest at three-month SOFR plus 2.80% (the “Class C 2025 Notes”); and $286,650 of Subordinated Notes due 2125 which do not bear interest (the “Subordinated 2025 Notes”). The Company indirectly retained the Class C 2025 Notes and Subordinated 2025 Notes.

Through June 20, 2029, all principal collections received on the underlying collateral may be used by the 2025 Issuer to purchase new collateral under the direction of the Investment Adviser, in its capacity as collateral manager of the 2025 Issuer and in accordance with the Company’s investment strategy and subject to customary conditions set forth in the documents governing the 2025 Debt Securitization, allowing the Company to maintain the initial leverage in the 2025 Debt Securitization. The 2025 Notes are scheduled to mature on July 20, 2037. The Subordinated 2025 Notes are due in 2125. The Class A-1 2025 Notes, Class A-2 2025 Notes and Class B 2025 Notes are included in the June 30, 2026 Consolidated Statement of Financial Condition as “Debt” of the Company. As of June 30, 2026, the Class C 2025 Notes and Subordinated 2025 Notes were eliminated in consolidation.

As of June 30, 2026 and September 30, 2025, there were 123 and 126 portfolio companies, respectively, with a total fair value of $1,125,962 and $1,139,381, respectively securing the 2025 Debt Securitization. The pool of loans in the 2025 Debt Securitization must meet certain requirements, including asset mix and concentration, collateral coverage, term, agency rating, minimum coupon, minimum spread and sector diversity requirements.

PNC Facility [Member]

Capital Stock, Long-Term Debt, and Other Securities [Abstract]

Long Term Debt, Title [Text Block]

PNC Facility

Long Term Debt, Structuring [Text Block]

PNC Facility: On July 8, 2022, the Company entered into a Revolving Credit and Security Agreement (the “PNC Facility”) with PNC Bank, National Association, as administrative agent, collateral agent, and a lender, and PNC Capital Markets LLC, as structuring agent. On June 30, 2025, all amounts outstanding under the PNC Facility were repaid, following which the agreements governing the PNC Facility were terminated. Prior to termination, the PNC Facility allowed the Company to borrow in an aggregate amount of up to $100,000, subject to leverage and borrowing base restrictions, with a stated maturity date of July 8, 2025 as the Company exercised the option on May 27, 2025 pursuant to the documents governing the PNC Facility to decrease the borrowing capacity under the PNC Facility from $250,000 to $100,000, effective as of June 3, 2025.

Prior to termination, the PNC Facility bore interest, at the Company’s election and depending on the currency of the borrowing, of either CORRA, SONIA, €STR, Daily Simple SOFR, Term SOFR, or the Base Rate (each, as defined in the PNC Facility) plus a margin ranging from 2.15% to 2.45%, depending on the degree of uncalled capital commitments coverage of the PNC Facility’s borrowing base versus the assets of GBDC 4 Funding securing the facility.

DB Credit Facility [Member]

Capital Stock, Long-Term Debt, and Other Securities [Abstract]

Long Term Debt, Title [Text Block]

DB Credit Facility

Long Term Debt, Structuring [Text Block]

On March 28, 2024 (the “DB Credit Facility Effective Date”), the Company and GBDC 4 Funding II entered into a loan financing and servicing agreement (as amended, the “DB Credit Facility”), with the Company, as equity holder and as servicer, the lenders from time to time parties thereto, Deutsche Bank AG, New York Branch, as facility agent, the other agents parties thereto, each of the entities from time to time party thereto as securitization subsidiaries and Deutsche Bank National Trust Company, as collateral agent and as collateral custodian. The period during which GBDC 4 Funding II may request drawdowns under the DB Credit Facility (the “DB Credit Facility Revolving Period”) commenced on the DB Credit Facility Effective Date and will continue through March 28, 2027 unless there is an earlier termination or event of default. The DB Credit Facility will mature on March 28, 2030, three years from the last day of the DB Credit Facility Revolving Period, unless terminated earlier in accordance with the DB Credit Facility. On June 13, 2025, the Company entered into an amendment to the DB Credit Facility that, among other things, reduced the applicable margin (a) effective during the DB Credit Facility Revolving Period from 2.35% to 1.75% and (b) effective after the DB Credit Facility Revolving Period from 2.85% to 2.25%. As of June 30, 2026, the DB Credit Facility allowed the Company to borrow in an aggregate amount of up to $300,000, subject to leverage and borrowing base restrictions.

As of June 30, 2026, the DB Credit Facility bears interest at the applicable base rate plus 1.75% per annum during the DB Credit Facility Revolving Period and 2.25% after the DB Credit Facility Revolving Period. The base rate under the DB Credit Facility is (i) the three-month CORRA plus an adjustment for a period of three months equal to 0.32138% with respect to any advances denominated in Canadian dollars, (ii) the three-month EURIBOR with respect to any advances denominated in euros, (iii) the three-month Bank Bill Swap Rate with respect to any advances denominated in Australian dollars, (iv) the daily simple SONIA with respect to any advances denominated in U.K. pound sterling plus an adjustment for a period of three months equal to 0.1193%, (v) the daily simple Swiss Average Rate Overnight with respect to any advances denominated in Swiss francs, (vi) the three-month Copenhagen Interbank Offered Rate with respect to any advances denominated in Danish krones, (vii) the three-month Bank Bill Benchmark Rate with respect to any advances denominated in New Zealand dollars, (viii) the three-month Norwegian Krone Interbank Offered Rate with respect to any advances denominated in Norwegian krone, (ix) the three-month Stockholm Interbank Offered Rate with respect to any advances denominated in Swedish krona and (x) the three-month term SOFR with respect to any other advances. Additionally, a syndication/agent fee is payable to the facility agent each quarter. In addition, a non-usage fee of 0.25% per annum is payable on the undrawn amount under the DB Credit Facility, and, during the DB Credit Facility Revolving Period, an additional fee based on unfunded commitments of the lenders could be payable if borrowings under the DB Credit Facility do not exceed a minimum utilization percentage threshold. A prepayment fee may be payable in the event of any permanent reduction in commitments of the DB Credit Facility in the amount of 0.50% or 0.25% of the amount of the reduction during the first or second year after the DB Credit Facility Effective Date, respectively.

The DB Credit Facility is secured by all of the assets held by GBDC 4 Funding II. GBDC 4 Funding II has made customary representations and warranties and is required to comply with various covenants, reporting requirements and other customary requirements for similar credit facilities. The borrowings of the Company, including under the DB Credit Facility, are subject to the leverage restrictions contained in the 1940 Act.

The Company transfers certain loans and debt securities it has originated or acquired from time to time to GBDC 4 Funding II through a sale and contribution agreement and causes GBDC 4 Funding II to originate or acquire loans, consistent with the Company’s investment objectives.

BNP Credit Facility [Member]

Capital Stock, Long-Term Debt, and Other Securities [Abstract]

Long Term Debt, Title [Text Block]

BNP Credit Facility

Long Term Debt, Structuring [Text Block]

On August 15, 2024 (the “BNP Credit Facility Effective Date”), the Company and GBDC 4 Funding III entered into a revolving credit and security agreement (the “BNP Credit Facility”), with the Company, as equity holder and as servicer, the lenders from time to time parties thereto, BNP Paribas, as administrative agent, each of the securitization subsidiaries from time to time parties thereto and Computershare Trust Company, N.A., as collateral agent. Under the BNP Credit Facility, the lenders have agreed to extend credit to GBDC 4 Funding III in an aggregate principal amount of up to $750,000 as of June 30, 2026. The period during which GBDC 4 Funding III may request drawdowns under the BNP Credit Facility (the “BNP Credit Facility Revolving Period”) commenced on the BNP Credit Facility Effective Date and will continue through August 15, 2027, unless there is an earlier termination due to an event of default. The BNP Credit Facility will mature on August 15, 2030, the first business day on or after the date that is the 36-month anniversary of the last day of the BNP Credit Facility Revolving Period, unless terminated earlier in accordance with the BNP Credit Facility. On November 26, 2025, the Company entered into an agreement to the documents governing the BNP Credit Facility that, among other things, reduced the applicable margin (a) effective during the BNP Credit Facility Reinvestment Period from 2.10% to 1.85% and (b) effective after the BNP Credit Facility Reinvestment Period from 2.35% to 2.10%.

As of June 30, 2026, borrowings under the BNP Credit Facility in United States Dollars will bear interest at the applicable base rate plus 1.85% per annum during the BNP Credit Facility Revolving Period, and 2.10% per annum after the BNP Credit Facility Revolving Period (the “BNP Credit Facility Applicable Margin”), and borrowings in eligible currencies other than United States Dollars will bear interest at the applicable base rate plus the BNP Credit Facility Applicable Margin. The base rate under the BNP Credit Facility is (i) term CORRA with respect to any advances denominated in Canadian dollars, (ii) the 3-month Euro Interbank Offered Rate with respect to any advances denominated in Euros, (iii) the 3-month Bank Bill Swap Rate with respect to any advances denominated in Australian dollars, (iv) the adjusted cumulative compound Sterling Overnight Index Average with respect to any advances denominated in U.K. pound sterling, (v) daily simple adjusted non-cumulative compound SARON with respect to any advances denominated in Swiss francs, (vi) 3-month Norwegian Interbank Offered Rate with respect to any advances denominated in Norwegian Krona, (vii) 3-month Stockholm Interbank Offered Rate with respect to any advances denominated in Swedish Krona and (viii) the term SOFR with respect to advances denominated in United States Dollars and any other advances.

The BNP Credit Facility is secured by all of the assets held by GBDC 4 Funding III. GBDC 4 Funding III has made customary representations and warranties and is required to comply with various covenants, reporting requirements and other customary requirements for similar credit facilities. The borrowings of the Company, including under the BNP Credit Facility, are subject to the leverage restrictions contained in the 1940 Act.

The Company transfers certain loans and debt securities it has originated or acquired from time to time to GBDC 4 Funding III through a sale and contribution agreement and causes GBDC 4 Funding III to originate or acquire loans, consistent with the Company’s investment objectives.

Advisor Revolver [Member]

Capital Stock, Long-Term Debt, and Other Securities [Abstract]

Long Term Debt, Title [Text Block]

Adviser Revolver

Long Term Debt, Structuring [Text Block]

The Company has entered into the Adviser Revolver with the Investment Adviser pursuant to which, as of June 30, 2026 and September 30, 2025, the Company was permitted to borrow up to $100,000 in U.S. dollars and certain agreed upon foreign currencies, with a maturity date of March 26, 2028. The Adviser Revolver bears an interest rate equal to the short-term Applicable Federal Rate (“AFR”). The short-term AFR as of June 30, 2026 was 3.8%.