Variable Interest Entities and Securitized Debt Obligations - Securitized Debt Obligations (Details) - USD ($) $ in Thousands	Dec. 31, 2022	Dec. 31, 2021
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Carrying Value	$ 1,138,749	$ 1,677,619
Collateral assets
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Principal Balance	1,569,622	2,278,100
Carrying Value	$ 1,541,540	$ 2,251,264
Weighted average interest rate of securitized debt obligations outstanding	3.90%	4.00%
Collateral assets | GPMT 2021-FL4 CRE CLO
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Principal Balance	$ 621,409	$ 621,409
Carrying Value	$ 607,354	$ 613,504
Weighted average interest rate of securitized debt obligations outstanding	3.70%	3.70%
Restricted cash		$ 0
Collateral assets | GPMT 2021-FL3 CRE CLO
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Principal Balance	$ 677,715	768,850
Carrying Value	$ 669,279	$ 763,607
Weighted average interest rate of securitized debt obligations outstanding	3.90%	3.90%
Restricted cash	$ 5,600	$ 10,400
Collateral assets | GPMT 2019-FL2 CRE CLO
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Principal Balance	270,498	617,119
Carrying Value	$ 264,907	$ 605,831
Weighted average interest rate of securitized debt obligations outstanding	4.20%	4.10%
Collateral assets | GPMT 2018-FL1 CRE CLO
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Principal Balance	$ 0	$ 270,722
Carrying Value	0	$ 268,322
Weighted average interest rate of securitized debt obligations outstanding		5.00%
Restricted cash		$ 0
Financing provided
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Principal Balance	1,141,740	1,684,763
Carrying Value	$ 1,138,749	$ 1,677,618
Weighted average interest rate of securitized debt obligations outstanding	1.80%	1.80%
Financing provided | GPMT 2021-FL4 CRE CLO
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Principal Balance	$ 502,564	$ 502,564
Carrying Value	$ 499,249	$ 498,117
Weighted average interest rate of securitized debt obligations outstanding	1.70%	1.70%
Financing provided | GPMT 2021-FL3 CRE CLO
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Principal Balance	$ 539,876	$ 630,818
Carrying Value	$ 539,892	$ 629,049
Weighted average interest rate of securitized debt obligations outstanding	1.70%	1.70%
Financing provided | GPMT 2019-FL2 CRE CLO
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Principal Balance	$ 99,300	$ 446,849
Carrying Value	$ 99,608	$ 445,920
Weighted average interest rate of securitized debt obligations outstanding	2.70%	1.80%
Financing provided | GPMT 2018-FL1 CRE CLO
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Principal Balance	$ 0	$ 104,532
Carrying Value	$ 0	$ 104,532
Weighted average interest rate of securitized debt obligations outstanding		2.80%