Annual report pursuant to Section 13 and 15(d)

Real Estate and Intangible Assets - Summary of Properties Acquired (Parenthetical) (Detail)

v2.4.1.9
Real Estate and Intangible Assets - Summary of Properties Acquired (Parenthetical) (Detail) (USD $)
In Thousands, unless otherwise specified
12 Months Ended 0 Months Ended 1 Months Ended 0 Months Ended
Dec. 31, 2014
sqft
Dec. 31, 2013
Oct. 22, 2014
Jul. 31, 2004
Oct. 31, 2005
Dec. 23, 2014
Buildings
Lease
Jul. 25, 2014
sqft
Nov. 30, 2014
Apr. 30, 2013
Business Acquisition [Line Items]                  
Lease expiration year 2016-11                
Lease renewal terms description The other tenants in the building have varying lease expirations from December 2015 to October 2018. No other tenants have renewal options.                
Area of property occupied by largest tenant 61,306us-gaap_AreaOfRealEstateProperty           81,371us-gaap_AreaOfRealEstateProperty    
Total Purchase Price $ 140,011good_PurchasePrice $ 134,438good_PurchasePrice              
Total investment in property 722,565us-gaap_RealEstateInvestmentPropertyAtCost 642,353us-gaap_RealEstateInvestmentPropertyAtCost              
Canton, North Carolina [Member]                  
Business Acquisition [Line Items]                  
Lease expiration year     2024-07            
Total Purchase Price 5,550good_PurchasePrice
/ us-gaap_BusinessAcquisitionAxis
= good_CantonNorthCarolinaMember
    5,200good_PurchasePrice
/ us-gaap_BusinessAcquisitionAxis
= good_CantonNorthCarolinaMember
         
Total investment in property               10,800us-gaap_RealEstateInvestmentPropertyAtCost
/ us-gaap_BusinessAcquisitionAxis
= good_CantonNorthCarolinaMember
 
Clintonville, WI [Member]                  
Business Acquisition [Line Items]                  
Total Purchase Price   3,250good_PurchasePrice
/ us-gaap_BusinessAcquisitionAxis
= good_ClintonvilleWiMember
    5,300good_PurchasePrice
/ us-gaap_BusinessAcquisitionAxis
= good_ClintonvilleWiMember
       
Total investment in property                 8,600us-gaap_RealEstateInvestmentPropertyAtCost
/ us-gaap_BusinessAcquisitionAxis
= good_ClintonvilleWiMember
Monroe, MI [Member]                  
Business Acquisition [Line Items]                  
Total Purchase Price $ 30,750good_PurchasePrice
/ us-gaap_BusinessAcquisitionAxis
= good_MonroeMember
               
Number of buildings acquired           2good_NumberOfBuildingsAcquired
/ us-gaap_BusinessAcquisitionAxis
= good_MonroeMember
     
Number of leases acquired           2good_NumberOfLeaseAgreementsAcquired
/ us-gaap_BusinessAcquisitionAxis
= good_MonroeMember