Name:	gpmt_RepurchaseAgreementCounterpartyPercentOfEquityAtRisk
Namespace Prefix:	gpmt_
Data Type:	dtr-types:percentItemType
Balance Type:	na
Period Type:	instant

Amount, including accrued interest, to be repaid to counterparty to repurchase agreement when carrying amount or market value if higher, of security or other asset sold under repurchase agreement exceeds 10 percent of total assets.

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 235
-SubTopic 10
-Name Accounting Standards Codification
-Section S50
-Paragraph 1
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147480738/235-10-S50-1

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Topic 860
-SubTopic 30
-Name Accounting Standards Codification
-Section 50
-Paragraph 7
-Subparagraph (d)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147481420/860-30-50-7

Reference 3: http://www.xbrl.org/2003/role/disclosureRef
-Topic 860
-SubTopic 30
-Name Accounting Standards Codification
-Section 45
-Paragraph 2
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147481444/860-30-45-2

Reference 4: http://www.xbrl.org/2003/role/disclosureRef
-Topic 235
-SubTopic 10
-Name Accounting Standards Codification
-Section S99
-Paragraph 1
-Subparagraph (SX 210.4-08(m)(1)(i))
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147480678/235-10-S99-1

Reference 5: http://www.xbrl.org/2003/role/disclosureRef
-Topic 235
-SubTopic 10
-Name Accounting Standards Codification
-Section S99
-Paragraph 1
-Subparagraph (SX 210.4-08(m)(1)(ii)(A)(ii))
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147480678/235-10-S99-1

Name:	us-gaap_AssetsSoldUnderAgreementsToRepurchaseRepurchaseLiability
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

The amount at risk under repurchase agreements is defined as the excess of carrying amount (or market value, if higher than the carrying amount or if there is no carrying amount) of the securities or other assets sold under agreement to repurchase, including accrued interest plus any cash or other assets on deposit to secure the repurchase obligation, over the amount of the repurchase liability (adjusted for accrued interest).

Name:	us-gaap_RepurchaseAgreementCounterpartyAmountAtRisk
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

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.

Name:	us-gaap_RepurchaseAgreementCounterpartyLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

Weighted average maturity (weighted by amount of the agreement) of repurchase agreements with the counterparty, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.