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Schedule III - Real Estate and Accumulated Depreciation - Change in Balance of Accumulated Depreciation 2 (Detail) - USD ($) $ in Millions | Dec. 31, 2020 | Dec. 31, 2019 | Dec. 31, 2018
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Real Estate Held for Sale |  |  |
SEC Schedule, 12-28, Real Estate Companies, Investment in Real Estate and Accumulated Depreciation [Line Items] |  |  |
Real estate held for sale, accumulated depreciation | $ 3.4 | $ 3.4 | $ 0.2

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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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Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

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- Details
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Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |