Commitments and Contingencies Unfunded Commitments (Details) - USD ($) $ in Thousands	9 Months Ended
Sep. 30, 2020	Dec. 31, 2019
Commitments and Contingencies Disclosure [Abstract]
Unfunded commitments	$ 596,440	$ 748,878
Weighted average future funding period on unfunded commitments	3 years

Weighted average future funding period of unfunded commitments for which the entity is obligated, to loans held-for-investment.

No definition available.

Name:	gpmt_WeightedAverageFutureFundingPeriodLoansHeldforInvestment
Namespace Prefix:	gpmt_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration