Quarterly report pursuant to Section 13 or 15(d)

Stockholders' Equity - Additional Information (Detail)

v2.4.1.9
Stockholders' Equity - Additional Information (Detail) (USD $)
 0 Months Ended 1 Months Ended 3 Months Ended
Mar. 28, 2015
Mar. 31, 2011
Mar. 31, 2015
Mar. 31, 2014
Dec. 31, 2014
Sep. 02, 2014
Class of Stock [Line Items]            
Common stock, shares issued     20,459,687us-gaap_CommonStockSharesIssued   19,589,606us-gaap_CommonStockSharesIssued  
Common stock, par value     $ 0.001us-gaap_CommonStockParOrStatedValuePerShare   $ 0.001us-gaap_CommonStockParOrStatedValuePerShare  
Shares intended to be offered pursuant to primary offering   3,000,000good_SharesIntendedToBeOfferedPursuantToPrimaryOffering        
Shares intended to be offered pursuant to senior common distribution reinvestment plan   500,000us-gaap_StockIssuedDuringPeriodSharesDividendReinvestmentPlan        
Shares of senior common stock sold in offering 927,994good_SharesOfSeniorCommonStockSoldInThisOngoingOffering   189,052good_SharesOfSeniorCommonStockSoldInThisOngoingOffering      
Senior common stock offering, gross proceeds     $ 18,135,000us-gaap_ProceedsFromIssuanceOfCommonStock $ 7,707,000us-gaap_ProceedsFromIssuanceOfCommonStock    
Shares of senior common stock issued     1,001,957good_SeniorCommonStockSharesIssued   809,411good_SeniorCommonStockSharesIssued  
Cantor Fitzgerald & Co [Member]            
Class of Stock [Line Items]            
Maximum aggregate sales price of shares to be issued under open market sale agreement           100,000,000good_MaximumAggregateSalesPriceOfSharesToBeIssuedUnderOpenMarketSaleAgreement
/ dei_LegalEntityAxis
= good_CantorFitzgeraldAndCompanyMember
Net proceeds under ATM program     15,100,000us-gaap_ProceedsFromIssuanceOrSaleOfEquity
/ dei_LegalEntityAxis
= good_CantorFitzgeraldAndCompanyMember 		     
Number of shares sold under open market sale agreement     2,700,000good_NumberOfSharesSoldUnderOpenMarketSaleAgreement
/ dei_LegalEntityAxis
= good_CantorFitzgeraldAndCompanyMember 		     
Gross proceeds of shares sold under open market sale agreement     47,800,000good_GrossProceedsOfSharesSoldUnderOpenMarketSaleAgreement
/ dei_LegalEntityAxis
= good_CantorFitzgeraldAndCompanyMember 		     
Maximum remaining capacity to sell common stock under open market sale agreement     52,200,000good_MaximumRemainingCapacityToSellCommonStockUnderOpenMarketSaleAgreement
/ dei_LegalEntityAxis
= good_CantorFitzgeraldAndCompanyMember 		     
Senior Common Stock [Member]            
Class of Stock [Line Items]            
Common stock, shares issued   3,500,000us-gaap_CommonStockSharesIssued
/ us-gaap_StatementClassOfStockAxis
= good_SeniorCommonStockMember 		       
Common stock, par value   0.001us-gaap_CommonStockParOrStatedValuePerShare
/ us-gaap_StatementClassOfStockAxis
= good_SeniorCommonStockMember 		       
Common stock shares issued, selling price   15.00good_CommonStockSharesIssuedSellingPrice
/ us-gaap_StatementClassOfStockAxis
= good_SeniorCommonStockMember 		$ 15.00good_CommonStockSharesIssuedSellingPrice
/ us-gaap_StatementClassOfStockAxis
= good_SeniorCommonStockMember 		     
Net proceeds after deducting underwriting discount and commission     2,600,000good_NetProceedsFromIssuanceInitialPublicOffering
/ us-gaap_StatementClassOfStockAxis
= good_SeniorCommonStockMember 		     
Senior common stock offering, gross proceeds 13,900,000us-gaap_ProceedsFromIssuanceOfCommonStock
/ us-gaap_StatementClassOfStockAxis
= good_SeniorCommonStockMember 		         
Distribution Reinvestment Plan [Member]            
Class of Stock [Line Items]            
Shares of senior common stock issued 27,861good_SeniorCommonStockSharesIssued
/ us-gaap_StatementClassOfStockAxis
= good_DistributionReinvestmentPlanMember 		  5,239good_SeniorCommonStockSharesIssued
/ us-gaap_StatementClassOfStockAxis
= good_DistributionReinvestmentPlanMember