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v3.19.2

Real Estate Dispositions, Held for Sale and Impairment Charges - Additional Information (Detail) - property | 6 Months Ended |
--- | --- | ---
Jun. 30, 2019 | Dec. 31, 2018
Real Estate Properties [Line Items] |  |
Number of properties sold | 1 |
Number of impaired assets, held and used | 0 |
Number of impaired assets sold, held for sale |  | 1
Maitland, Florida |  |
Real Estate Properties [Line Items] |  |
Number of real estate properties, held for sale |  | 1
Disposed of by sale |  |
Real Estate Properties [Line Items] |  |
Number of properties sold | 1 |

X
- Definition Number of real estate properties held for sale. + References No definition available. + Details
Name: | good_NumberOfRealEstatePropertiesHeldForSale
Namespace Prefix: | good_
Data Type: | xbrli:integerItemType
Balance Type: | na
Period Type: | instant

X
- Definition Number of Impaired Assets, Held and Used + References No definition available. + Details
Name: | good_NumberofImpairedAssetsHeldandUsed
Namespace Prefix: | good_
Data Type: | xbrli:integerItemType
Balance Type: | na
Period Type: | instant

X
- Definition Number of Impaired Assets Sold, Held for Sale + References No definition available. + Details
Name: | good_NumberofImpairedAssetsSoldHeldforSale
Namespace Prefix: | good_
Data Type: | xbrli:integerItemType
Balance Type: | na
Period Type: | instant

X
- Definition Number of Non-Core Properties Sold + References No definition available. + Details
Name: | good_NumberofNonCorePropertiesSold
Namespace Prefix: | good_
Data Type: | xbrli:integerItemType
Balance Type: | na
Period Type: | duration

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

X
- Details
Name: | srt_StatementGeographicalAxis=good_MaitlandFloridaMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_DisposalGroupClassificationAxis=us-gaap_DisposalGroupDisposedOfBySaleNotDiscontinuedOperationsMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |