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Real Estate and Intangible Assets - Additional Information (Detail) - property | Dec. 31, 2019 | Dec. 31, 2018
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Real Estate [Abstract] |  |
Number of properties acquired | 18 | 5

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- Definition Number of properties acquired. + References No definition available. + Details
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Namespace Prefix: | good_
Data Type: | xbrli:integerItemType
Balance Type: | na
Period Type: | instant

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- References No definition available. + Details
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Namespace Prefix: | us-gaap_
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Balance Type: | na
Period Type: | duration