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Real Estate and Intangible Assets - Leasing Activity (Details) $ in Thousands | 12 Months Ended
--- | ---
Dec. 31, 2016 USD ($) ft² | Dec. 31, 2015 USD ($) ft²
Real Estate Properties [Line Items] |  |
Square Footage | ft² | 551,335 | 890,092
Annualized GAAP Rent | $ 2,478 | $ 7,514
Tenant Improvement | 1,244 | 2,881
Leasing Commissions | $ 436 | $ 1,925
Weighted Average [Member] |  |
Real Estate Properties [Line Items] |  |
Lease Term | 3 years 10 months 24 days | 7 years 3 months 18 days
Minimum [Member] |  |
Real Estate Properties [Line Items] |  |
Lease Term | 1 year | 1 year
Maximum [Member] |  |
Real Estate Properties [Line Items] |  |
Lease Term | 7 years 8 months 12 days | 12 years 4 months 24 days

X
- 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

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: | us-gaap_RangeAxis=us-gaap_WeightedAverageMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

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

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