Nonaccrual Loans (Details)	9 Months Ended
Sep. 30, 2020
Receivables [Abstract]
Threshold period when delinquent loans are placed on nonaccrual status	90 days

Threshold period delinquent for financing receivable to place on nonaccrual status, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

No definition available.

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