Annual report pursuant to Section 13 and 15(d)

Related-Party Transactions - Additional Information (Detail)

v2.4.1.9
Related-Party Transactions - Additional Information (Detail) (USD $)
 12 Months Ended
Dec. 31, 2014
Dec. 31, 2013
Dec. 31, 2012
Related Party Transaction [Line Items]      
Due to Adviser and Administrator $ 916,000us-gaap_DueToRelatedPartiesCurrentAndNoncurrent [1] $ 1,360,000us-gaap_DueToRelatedPartiesCurrentAndNoncurrent [1]  
Annual base management fee, in percentage of stockholders' equity, in excess of recorded value of preferred stock 2.00%good_AnnualBaseManagementFeeInPercentageOfStockholdersEquityInExcessOfRecordedValueOfPreferredStock    
Base management fee 2,798,000good_BaseManagementFee [1] 2,014,000good_BaseManagementFee [1] 1,467,000good_BaseManagementFee [1]
Credit to base management fee 200,000us-gaap_ManagementFeesBaseRevenue    
Pre-incentive quarterly fee FFO in percentage of common stockholders' equity that will reward the Adviser 1.75%good_IncentiveFeePerAdvisoryAgreement    
Pre-incentive annual fee FFO in percentage of common stockholders' equity that will reward the Adviser 7.00%good_AnnualizedIncentiveFeePerAdvisoryAgreement    
Amount to be paid to Adviser in percentage of pre-incentive fee condition one 100.00%good_AmountToBePaidToAdviserInPercentageOfPreIncentiveFeeConditionOne    
Pre-incentive fee in percentage of common stockholders' equity that awards Adviser hundred percent of amount of pre-incentive fee, maximum percentage 2.1875%good_PreIncentiveFeeInPercentageOfCommonStockholdersEquityThatAwardsAdviserHundredPercentOfAmountOfPreIncentiveFeeMaximumPercentage    
Amount to be paid to Adviser in percentage of pre-incentive fee condition two 20.00%good_AmountToBePaidToAdviserInPercentageOfPreIncentiveFeeConditionTwo    
Pre-incentive fee in percentage of common stockholders' equity that awards the Adviser 20% of the amount of the pre-incentive fee, minimum percentage 2.1875%good_PreIncentiveFeeInPercentageOfCommonStockholdersEquityThatAwardsAdviserTwentyPercentOfAmountOfPreIncentiveFeeMinimumPercentage    
Incentive fee 4,305,000good_IncentiveFee [1] 4,201,000good_IncentiveFee [1] 3,569,000good_IncentiveFee [1]
Credits related to unconditional and irrevocable voluntary waivers issued by the Adviser 3,013,000good_CreditToIncentiveFee [1] 3,457,000good_CreditToIncentiveFee [1] 2,221,000good_CreditToIncentiveFee [1]
Related-party transactions incentive fee, net 1,300,000good_RelatedPartyTransactionsIncentiveFeeNet 700,000good_RelatedPartyTransactionsIncentiveFeeNet 1,400,000good_RelatedPartyTransactionsIncentiveFeeNet
Administration fee 1,526,000good_AdministrationFee [1] 1,467,000good_AdministrationFee [1] 1,118,000good_AdministrationFee [1]
Dealer manager fee in percentage of gross proceeds of shares of Senior Common Stock sold 7.00%good_DealerManagerFeeInPercentageOfGrossProceedsOfSharesOfSeniorCommonStockSold    
Sales commission fee in percentage of gross proceeds of shares of Senior Common Stock sold 3.00%good_SalesCommissionFeeInPercentageOfGrossProceedsOfSharesOfSeniorCommonStockSold    
Agreement termination date Aug. 31, 2015    
Fees paid 100,000good_PaymentOfFinancingFee 100,000good_PaymentOfFinancingFee  
Total secured mortgages subject to financing fee 52,100,000us-gaap_SecuredDebt 76,300,000us-gaap_SecuredDebt  
Financing fee on secured mortgages percentage 0.27%good_SecuredMortgagesFinancingFeePercent 0.18%good_SecuredMortgagesFinancingFeePercent  
Minimum [Member]      
Related Party Transaction [Line Items]      
Percentage of the amount of the mortgage 0.15%good_PercentageOfFinancingFeeToAmountOfMortgage
/ us-gaap_RangeAxis
= us-gaap_MinimumMember 		   
Maximum [Member]      
Related Party Transaction [Line Items]      
Percentage of the amount of the mortgage 1.00%good_PercentageOfFinancingFeeToAmountOfMortgage
/ us-gaap_RangeAxis
= us-gaap_MaximumMember 		   
Dealer Manager [Member]      
Related Party Transaction [Line Items]      
Payments made to the Dealer Manager pursuant to Dealer Manager Agreement $ 600,000us-gaap_PaymentsForAdvanceToAffiliate
/ us-gaap_RelatedPartyTransactionsByRelatedPartyAxis
= good_DealerManagerMember 		$ 300,000us-gaap_PaymentsForAdvanceToAffiliate
/ us-gaap_RelatedPartyTransactionsByRelatedPartyAxis
= good_DealerManagerMember 		$ 200,000us-gaap_PaymentsForAdvanceToAffiliate
/ us-gaap_RelatedPartyTransactionsByRelatedPartyAxis
= good_DealerManagerMember
Agreement termination date Mar. 28, 2015    
Agreement termination on Senior Common Stock shares issued 3,000,000good_AgreementTerminationOnSeniorCommonStockSharesIssued
/ us-gaap_RelatedPartyTransactionsByRelatedPartyAxis
= good_DealerManagerMember 		   
[1] Refer to Note 2 "Related-Party Transactions"