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Real Estate and Intangible Assets - Additional Information (Detail) - property | Dec. 31, 2017 | Dec. 31, 2016
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Real Estate [Abstract] |  |
Number of properties acquired | 7 | 3

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

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- References No definition available. + Details
Name: | us-gaap_RealEstateAbstract
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration