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Real Estate and Intangible Assets - Summary of Lease on Property (Detail) $ in Thousands	3 Months Ended
	Mar. 31, 2018 USD ($) ft²
Real Estate Properties [Line Items]
Aggregate Square Footage | ft²	34,441
Aggregate Annualized GAAP Rent	$ 97
Aggregate Leasing Commissions	$ 14
Weighted average
Real Estate Properties [Line Items]
Lease Term	3 years 7 months 6 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

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: