Variable Interest Entities and Securitized Debt Obligations - Securitized Debt Obligations (Details) - USD ($)	3 Months Ended	6 Months Ended
Jun. 30, 2023	Jun. 30, 2023	Mar. 16, 2023	Dec. 31, 2022
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Carrying Value	$ 999,781,000	$ 999,781,000		$ 1,138,749,000
Rental income	444,000	444,000
Cash collateral for repurchase agreements and securities activity	1,400,000	1,400,000		1,400,000
Restricted cash
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Cash collateral for repurchase agreements and securities activity	39,600,000	39,600,000
GPMT 2021-FL4 CRE CLO \| Restricted cash
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Cash collateral for repurchase agreements and securities activity	39,600,000	39,600,000		0
GPMT 2021-FL3 CRE CLO \| Restricted cash
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Cash collateral for repurchase agreements and securities activity	0	0		5,600,000
GPMT 2019-FL2 CRE CLO \| Restricted cash
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Cash collateral for repurchase agreements and securities activity	0	0
Collateral assets
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Principal Balance	1,259,209,000	1,259,209,000		1,569,622,000
Carrying Value	$ 1,223,998,000	$ 1,223,998,000		$ 1,541,540,000
Weighted average interest rate of securitized debt obligations outstanding	3.80%	3.80%		3.90%
Collateral assets \| GPMT 2021-FL4 CRE CLO
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Principal Balance	$ 621,409,000	$ 621,409,000		$ 621,409,000
Carrying Value	$ 602,588,000	$ 602,588,000		$ 607,354,000
Weighted average interest rate of securitized debt obligations outstanding	3.80%	3.80%		3.70%
Collateral assets \| GPMT 2021-FL3 CRE CLO
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Principal Balance	$ 637,800,000	$ 637,800,000		$ 677,715,000
Carrying Value	$ 621,410,000	$ 621,410,000		$ 669,279,000
Weighted average interest rate of securitized debt obligations outstanding	3.80%	3.80%		3.90%
Collateral assets \| GPMT 2019-FL2 CRE CLO
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Principal Balance	$ 0	$ 0	$ 269,300,000	$ 270,498,000
Carrying Value	0	0		$ 264,907,000
Weighted average interest rate of securitized debt obligations outstanding				4.20%
Financing provided
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Principal Balance	1,002,525,000	1,002,525,000		$ 1,141,740,000
Carrying Value	$ 999,781,000	$ 999,781,000		$ 1,138,749,000
Weighted average interest rate of securitized debt obligations outstanding	1.70%	1.70%		1.80%
Financing provided \| GPMT 2021-FL4 CRE CLO
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Principal Balance	$ 502,564,000	$ 502,564,000		$ 502,564,000
Carrying Value	$ 499,820,000	$ 499,820,000		$ 499,249,000
Weighted average interest rate of securitized debt obligations outstanding	1.70%	1.70%		1.70%
Financing provided \| GPMT 2021-FL3 CRE CLO
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Principal Balance	$ 499,961,000	$ 499,961,000		$ 539,876,000
Carrying Value	$ 499,961,000	$ 499,961,000		$ 539,892,000
Weighted average interest rate of securitized debt obligations outstanding	1.80%	1.80%		1.70%
Financing provided \| GPMT 2019-FL2 CRE CLO
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Principal Balance	$ 0	$ 0		$ 99,300,000
Carrying Value	$ 0	$ 0	$ 98,100,000	$ 99,608,000
Weighted average interest rate of securitized debt obligations outstanding				2.70%

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- Definition

Obligation reflecting another party's right to economic interests in another entity or trust under a contractual right. For example, an investor may purchase a beneficial interest in a securitization transaction.

+ References

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- Definition

Carrying amount as of the balance sheet date of cash collateral held for borrowed securities, for which the cash is restricted as to withdrawal or usage.

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 944
-SubTopic 210
-Name Accounting Standards Codification
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Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 808
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- Definition

Weighted average interest rate of debt outstanding.

+ References

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- Definition

Amount of operating lease income from lease payments and variable lease payments paid and payable to lessor. Includes, but is not limited to, variable lease payments not included in measurement of lease receivable.

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