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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

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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

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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

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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

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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

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Data Type:	xbrli:stringItemType
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Period Type:	duration

X

- Details

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Namespace Prefix:
Data Type:	na
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Period Type:

X

- Details

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