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Real Estate and Intangible Assets - Summary of Lease on Property (Detail) $ in Thousands | 6 Months Ended | 12 Months Ended
--- | --- | ---
Jun. 30, 2019 USD ($) ft² | Jun. 30, 2018 USD ($) ft² | Dec. 31, 2018 USD ($) | Dec. 31, 2017 USD ($)
Real Estate Properties [Line Items] |  |  |  |
Aggregate Square Footage | ft² | 230,264 | 34,441 |  |
Aggregate Annualized GAAP Rent | $ 3,366 | $ 97 |  |
Aggregate Tenant Improvement |  |  | $ 727 | $ 0
Aggregate Leasing Commissions | $ 470 | $ 14 |  |
Weighted average |  |  |  |
Real Estate Properties [Line Items] |  |  |  |
Weighted Average Lease Term | 8 years 10 months 6 days | 3 years 7 months 6 days |  |

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- Definition Annualized straight line rent on leased property. + References No definition available. + Details
Name: | good_AnnualizedStraightLineRentOnLeasedProperty
Namespace Prefix: | good_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

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- Definition Area of leased property. + References No definition available. + Details
Name: | good_AreaOfLeasedProperty
Namespace Prefix: | good_
Data Type: | num:areaItemType
Balance Type: | na
Period Type: | duration

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- Definition Lease term available for lease. + References No definition available. + Details
Name: | good_LeaseTermAvailableForLease
Namespace Prefix: | good_
Data Type: | xbrli:durationItemType
Balance Type: | na
Period Type: | duration

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- Definition Amount of cash outflow for lease commissions incurred. + References No definition available. + Details
Name: | us-gaap_PaymentsForLeaseCommissions
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

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- Definition Amount of cash outflow for the allowance granted to lessee and/or direct costs incurred by lessor used to prepare the leased premises for tenant's occupancy. + References No definition available. + Details
Name: | us-gaap_PaymentsForTenantImprovements
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

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

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- Details
Name: | srt_RangeAxis=srt_WeightedAverageMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |