Annual report pursuant to Section 13 and 15(d)

Real Estate and Intangible Assets - Additional Information (Detail)

v2.4.1.9
Real Estate and Intangible Assets - Additional Information (Detail) (USD $)
 0 Months Ended 12 Months Ended 0 Months Ended
Jun. 11, 2014
Dec. 31, 2014
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Dec. 31, 2013
Dec. 31, 2012
Oct. 22, 2014
Oct. 27, 2014
Option
Dec. 01, 2014
Nov. 14, 2014
Oct. 16, 2014
Option
May 06, 2014
Dec. 24, 2014
Jul. 07, 2014
Oct. 30, 2014
Jul. 25, 2014
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Real Estate Properties [Line Items]                            
Leasing commissions paid   $ 1,159,000us-gaap_PaymentsForLeaseCommissions $ 1,041,000us-gaap_PaymentsForLeaseCommissions $ 1,738,000us-gaap_PaymentsForLeaseCommissions                    
Tenant improvements payable   3,300,000good_TenantImprovementsPayable                        
Extended additional period of lease 3 years                          
Lease expiration period, month and year   2016-11                        
Annual increase in lease rental 14.00%good_AnnualIncreaseInLeaseRentalPercentage                          
Deferred rent, receivables   21,728,000us-gaap_DeferredRentReceivablesNet 18,905,000us-gaap_DeferredRentReceivablesNet                      
Finance for expansion project on property   5,600,000us-gaap_MortgageLoansOnRealEstate                       5,600,000us-gaap_MortgageLoansOnRealEstate
Expansion of our property, area   61,306us-gaap_AreaOfRealEstateProperty                       81,371us-gaap_AreaOfRealEstateProperty
Total annual property taxes for all properties not paid by tenants   12,600,000us-gaap_RealEstateTaxExpense                        
2013 Real Estate Activity [Member]                            
Real Estate Properties [Line Items]                            
Number of properties acquired     7good_NumberOfPropertiesAcquired
/ us-gaap_RealEstatePropertiesAxis
= good_TwoThousandThirteenNewRealEstateActivityMember 		                     
Number of properties expanded     1good_NumberOfPropertiesExpanded
/ us-gaap_RealEstatePropertiesAxis
= good_TwoThousandThirteenNewRealEstateActivityMember 		                     
2014 Real Estate Activity [Member]                            
Real Estate Properties [Line Items]                            
Number of properties acquired   11good_NumberOfPropertiesAcquired
/ us-gaap_RealEstatePropertiesAxis
= good_TwoThousandFourteenNewRealEstateActivityMember 		                       
Number of acquisitions completed   10good_NumberAcquisitionsCompleted
/ us-gaap_RealEstatePropertiesAxis
= good_TwoThousandFourteenNewRealEstateActivityMember 		                       
Number of properties expanded   1good_NumberOfPropertiesExpanded
/ us-gaap_RealEstatePropertiesAxis
= good_TwoThousandFourteenNewRealEstateActivityMember 		                       
Canton, North Carolina [Member]                            
Real Estate Properties [Line Items]                            
Extended additional period of lease         10 years                  
Extended lease expiration date         2034-09                  
Lease expiration period, month and year         2024-07                  
Annualized straight line rent escalations         1,400,000us-gaap_StraightLineRentAdjustments
/ us-gaap_BusinessAcquisitionAxis
= good_CantonNorthCarolinaMember 		                 
Lease Term   20 years                        
Finance for expansion project on property         5,500,000us-gaap_MortgageLoansOnRealEstate
/ us-gaap_BusinessAcquisitionAxis
= good_CantonNorthCarolinaMember 		                 
Area of additional real estate property         150,000good_AdditionalAreaOfRealEstatePropertyLeased
/ us-gaap_BusinessAcquisitionAxis
= good_CantonNorthCarolinaMember 		                 
Canton, North Carolina [Member] | Previous Lease [Member]                            
Real Estate Properties [Line Items]                            
Annualized straight line rent escalations         600,000us-gaap_StraightLineRentAdjustments
/ us-gaap_BusinessAcquisitionAxis
= good_CantonNorthCarolinaMember
/ us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis
= good_PreviousLeaseMember 		                 
South Hadley Massachusetts [Member]                            
Real Estate Properties [Line Items]                            
Extended additional period of lease           2 years                
Extended lease expiration date           2017-01                
Lease expiration period, month and year           2015-01                
Annualized straight line rent escalations           300,000us-gaap_StraightLineRentAdjustments
/ us-gaap_BusinessAcquisitionAxis
= good_SouthHadleyMassachusettsMember 		               
Number of options for renewal of lease           2good_NumberOfOptionsForRenewalOfLease
/ us-gaap_BusinessAcquisitionAxis
= good_SouthHadleyMassachusettsMember 		               
Number of years under additional periods for lease renewal option           1 year                
Maple Heights, Ohio Industrial Building [Member]                            
Real Estate Properties [Line Items]                            
Amount of annualized straight line rents over the life of the lease             30,000us-gaap_OperatingLeasesIncomeStatementMinimumLeaseRevenue
/ us-gaap_BusinessAcquisitionAxis
= good_MapleHeightsOhioIndustrialBuildingMember 		             
Leasing commissions paid             10,000us-gaap_PaymentsForLeaseCommissions
/ us-gaap_BusinessAcquisitionAxis
= good_MapleHeightsOhioIndustrialBuildingMember 		             
Expansion of our property, area             7,125us-gaap_AreaOfRealEstateProperty
/ us-gaap_BusinessAcquisitionAxis
= good_MapleHeightsOhioIndustrialBuildingMember 		             
Lease term             3 years              
Concord Township, Ohio [Member]                            
Real Estate Properties [Line Items]                            
Extended additional period of lease               5 years            
Deferred rent, uncollectible               2,500,000us-gaap_DeferredRentCreditCurrent
/ us-gaap_BusinessAcquisitionAxis
= good_ConcordTownshipOhioMember 		           
Concord Township, Ohio [Member] | General and Administrative Expense [Member]                            
Real Estate Properties [Line Items]                            
Deferred rent, receivables               800,000us-gaap_DeferredRentReceivablesNet
/ us-gaap_BusinessAcquisitionAxis
= good_ConcordTownshipOhioMember
/ us-gaap_IncomeStatementLocationAxis
= us-gaap_GeneralAndAdministrativeExpenseMember 		           
Concord Township, Ohio [Member] | Prior To Amendment [Member]                            
Real Estate Properties [Line Items]                            
Original lease expiration               2034-08            
Amount of annualized straight line rents over the life of the lease               1,700,000us-gaap_OperatingLeasesIncomeStatementMinimumLeaseRevenue
/ us-gaap_BusinessAcquisitionAxis
= good_ConcordTownshipOhioMember
/ us-gaap_StatementScenarioAxis
= good_PriorToAmendmentMember 		           
Concord Township, Ohio [Member] | After Amendment [Member]                            
Real Estate Properties [Line Items]                            
Original lease expiration               2024-12            
Amount of annualized straight line rents over the life of the lease               1,400,000us-gaap_OperatingLeasesIncomeStatementMinimumLeaseRevenue
/ us-gaap_BusinessAcquisitionAxis
= good_ConcordTownshipOhioMember
/ us-gaap_StatementScenarioAxis
= good_AfterAmendmentMember 		           
Richmond, Virginia [Member]                            
Real Estate Properties [Line Items]                            
Leasing commissions paid                 100,000us-gaap_PaymentsForLeaseCommissions
/ us-gaap_BusinessAcquisitionAxis
= good_RichmondVirginiaMember 		         
Tenant improvements payable                 0good_TenantImprovementsPayable
/ us-gaap_BusinessAcquisitionAxis
= good_RichmondVirginiaMember 		         
Annualized straight line rent escalations                 200,000us-gaap_StraightLineRentAdjustments
/ us-gaap_BusinessAcquisitionAxis
= good_RichmondVirginiaMember 		         
Lease Term                 3 years          
Lease early termination option description   The tenant has an early termination option at any time after July 31, 2016                        
Lease description   The tenant is subject to a termination payment of approximately $0.03 million if they exercise this option.                        
Termination payment                 30,000us-gaap_GainLossOnContractTermination
/ us-gaap_BusinessAcquisitionAxis
= good_RichmondVirginiaMember 		         
Number of options for renewal of lease                 1good_NumberOfOptionsForRenewalOfLease
/ us-gaap_BusinessAcquisitionAxis
= good_RichmondVirginiaMember 		         
Lease renewal period                 3 years          
Newburyport, Massachusetts [Member]                            
Real Estate Properties [Line Items]                            
Lease expiration date   Apr. 30, 2015                        
Burnsville, MN [Member]                            
Real Estate Properties [Line Items]                            
Leasing commissions paid   500,000us-gaap_PaymentsForLeaseCommissions
/ us-gaap_StatementGeographicalAxis
= good_BurnsvilleMinnesotaMember 		                       
Extended additional period of lease 8 years                          
Extended lease expiration date 2023-01                          
Lease expiration period, month and year 2015-06                          
Annualized straight line rent escalations 1,000,000us-gaap_StraightLineRentAdjustments
/ us-gaap_StatementGeographicalAxis
= good_BurnsvilleMinnesotaMember 		                         
Austin, Texas [Member]                            
Real Estate Properties [Line Items]                            
Option to extend lease term                   8 years        
Original lease expiration                   2015-06        
Extended lease expiration                   2022-06        
Amount of annualized straight line rents over the life of the lease                   700,000us-gaap_OperatingLeasesIncomeStatementMinimumLeaseRevenue
/ us-gaap_StatementGeographicalAxis
= good_AustinTxMember 		       
Leasing commissions paid   200,000us-gaap_PaymentsForLeaseCommissions
/ us-gaap_StatementGeographicalAxis
= good_AustinTxMember 		                       
Tenant improvements payable   1,300,000good_TenantImprovementsPayable
/ us-gaap_StatementGeographicalAxis
= good_AustinTxMember 		                       
Akron, Ohio [Member]                            
Real Estate Properties [Line Items]                            
Option to extend lease term                     5 years      
Original lease expiration                     2015-03 2015-03    
Extended lease expiration                     2020-03 2020-03    
Leasing commissions paid                       200,000us-gaap_PaymentsForLeaseCommissions
/ us-gaap_StatementGeographicalAxis
= good_AkronOhioMember 		   
Tenant improvements payable   900,000good_TenantImprovementsPayable
/ us-gaap_StatementGeographicalAxis
= good_AkronOhioMember 		                100,000good_TenantImprovementsPayable
/ us-gaap_StatementGeographicalAxis
= good_AkronOhioMember 		     
Extended additional period of lease                     5 years      
Annualized straight line rent escalations                     100,000good_AnnualizedStraightLineRentEscalations
/ us-gaap_StatementGeographicalAxis
= good_AkronOhioMember 		600,000good_AnnualizedStraightLineRentEscalations
/ us-gaap_StatementGeographicalAxis
= good_AkronOhioMember 		   
Tewksbury Massachusetts [Member]                            
Real Estate Properties [Line Items]                            
Lease expiration date   May 31, 2017                        
Tewksbury Massachusetts [Member] | Prior To Amendment [Member]                            
Real Estate Properties [Line Items]                            
Amount of annualized straight line rents over the life of the lease                         900,000us-gaap_OperatingLeasesIncomeStatementMinimumLeaseRevenue
/ us-gaap_StatementGeographicalAxis
= good_TewksburyMassachusettsMember
/ us-gaap_StatementScenarioAxis
= good_PriorToAmendmentMember 		 
Tewksbury Massachusetts [Member] | After Amendment [Member]                            
Real Estate Properties [Line Items]                            
Amount of annualized straight line rents over the life of the lease                         400,000us-gaap_OperatingLeasesIncomeStatementMinimumLeaseRevenue
/ us-gaap_StatementGeographicalAxis
= good_TewksburyMassachusettsMember
/ us-gaap_StatementScenarioAxis
= good_AfterAmendmentMember 		 
Bolingbrook Illinois [Member]                            
Real Estate Properties [Line Items]                            
Leasing commissions paid             200,000us-gaap_PaymentsForLeaseCommissions
/ us-gaap_StatementGeographicalAxis
= good_BolingbrookIllinoisMember 		             
Tenant improvements payable             500,000good_TenantImprovementsPayable
/ us-gaap_StatementGeographicalAxis
= good_BolingbrookIllinoisMember 		             
Extended additional period of lease             5 years              
Annualized straight line rent escalations             $ 200,000good_AnnualizedStraightLineRentEscalations
/ us-gaap_StatementGeographicalAxis
= good_BolingbrookIllinoisMember 		             
Number of options for renewal of lease             1good_NumberOfOptionsForRenewalOfLease
/ us-gaap_StatementGeographicalAxis
= good_BolingbrookIllinoisMember 		             
Lease term             7 years              
Percentage of building occupied by tenant             38.00%good_PercentageOfBuildingOccupiedByTenant
/ us-gaap_StatementGeographicalAxis
= good_BolingbrookIllinoisMember