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Schedule III - Real Estate and Accumulated Depreciation - Change in Balance of Real Estate 2 (Detail) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Real Estate Held for Sale
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Real estate held for sale, gross	$ 12,997	$ 11,454	$ 1,900

X

- Definition

Disposal group including discontinued operation real estate investment property at cost.

+ References

No definition available.

+ Details

Name:	good_DisposalGroupIncludingDiscontinuedOperationRealEstateInvestmentPropertyAtCost
Namespace Prefix:	good_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

X

- Definition

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

No definition available.

+ Details

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

X

- Details

Name:	us-gaap_RealEstatePropertiesAxis=good_RealEstateHeldForSaleMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: