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Schedule III - Real Estate and Accumulated Depreciation - Change in Balance of Accumulated Depreciation 2 (Detail) - USD ($) $ in Thousands | Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2015
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Real Estate Held for Sale |  |  |
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items] |  |  |
Real estate held for sale, accumulated depreciation | $ 3,970 | $ 2,668 | $ 800

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- Definition Disposal group including discontinued operation real estate investment property accumulated depreciation. + References No definition available. + Details
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Namespace Prefix: | good_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

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

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