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Real Estate and Intangible Assets - Summary of Lease on Property (Detail) $ in Thousands | 9 Months Ended
--- | ---
Sep. 30, 2017 USD ($) ft² | Sep. 30, 2016 USD ($) ft²
Real Estate Properties [Line Items] |  |
Aggregate Square Footage | ft² | 577,471 | 460,017
Aggregate Annualized GAAP Rent | $ 4,062 | $ 1,475
Aggregate Tenant Improvement | 1,181 | 333
Aggregate Leasing Commissions | $ 475 | $ 221
Weighted average |  |
Real Estate Properties [Line Items] |  |
Lease Term | 8 years 10 months 24 days | 2 years 9 months 18 days
Minimum |  |
Real Estate Properties [Line Items] |  |
Lease Term | 1 year | 1 year
Maximum |  |
Real Estate Properties [Line Items] |  |
Lease Term | 11 years 3 months | 7 years 3 months 18 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

X
- 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

X
- 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

X
- 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

X
- 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
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Namespace Prefix: |
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Period Type: |

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- Details
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Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

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- Details
Name: | us-gaap_RangeAxis=us-gaap_MaximumMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |