Annual report pursuant to Section 13 and 15(d)

Commitments and Contingencies - Future Minimum Rental Payments Due under Terms of Leases (Detail)

v2.4.1.9
Commitments and Contingencies - Future Minimum Rental Payments Due under Terms of Leases (Detail) (USD $)
In Thousands, unless otherwise specified
 12 Months Ended
Dec. 31, 2014
Operating Leased Assets [Line Items]  
For the year ended December 31, 2016 $ 68,669us-gaap_OperatingLeasesFutureMinimumPaymentsDueInTwoYears
For the year ended December 31, 2017 68,290us-gaap_OperatingLeasesFutureMinimumPaymentsDueInThreeYears
For the year ended December 31, 2018 67,983us-gaap_OperatingLeasesFutureMinimumPaymentsDueInFourYears
For the year ended December 31, 2019 68,405us-gaap_OperatingLeasesFutureMinimumPaymentsDueInFiveYears
Ground Leases [Member]  
Operating Leased Assets [Line Items]  
For the year ended December 31, 2015 429us-gaap_OperatingLeasesFutureMinimumPaymentsRemainderOfFiscalYear
/ us-gaap_PropertySubjectToOrAvailableForOperatingLeaseAxis
= good_GroundLeasesMember
For the year ended December 31, 2016 429us-gaap_OperatingLeasesFutureMinimumPaymentsDueInTwoYears
/ us-gaap_PropertySubjectToOrAvailableForOperatingLeaseAxis
= good_GroundLeasesMember
For the year ended December 31, 2017 432us-gaap_OperatingLeasesFutureMinimumPaymentsDueInThreeYears
/ us-gaap_PropertySubjectToOrAvailableForOperatingLeaseAxis
= good_GroundLeasesMember
For the year ended December 31, 2018 433us-gaap_OperatingLeasesFutureMinimumPaymentsDueInFourYears
/ us-gaap_PropertySubjectToOrAvailableForOperatingLeaseAxis
= good_GroundLeasesMember
For the year ended December 31, 2019 433us-gaap_OperatingLeasesFutureMinimumPaymentsDueInFiveYears
/ us-gaap_PropertySubjectToOrAvailableForOperatingLeaseAxis
= good_GroundLeasesMember
For the year ended December 31, 2020 433good_OperatingLeasesFutureMinimumPaymentsDueInSixYears
/ us-gaap_PropertySubjectToOrAvailableForOperatingLeaseAxis
= good_GroundLeasesMember
Thereafter 4,096good_OperatingLeasesFutureMinimumPaymentsDueInSixYearsAndThereafter
/ us-gaap_PropertySubjectToOrAvailableForOperatingLeaseAxis
= good_GroundLeasesMember
Ground Leases [Member] | Tulsa, OK [Member]  
Operating Leased Assets [Line Items]  
Lease End Date 2021-04
For the year ended December 31, 2015 169us-gaap_OperatingLeasesFutureMinimumPaymentsRemainderOfFiscalYear
/ us-gaap_PropertyPlantAndEquipmentByTypeAxis
= good_TulsaOkMember
/ us-gaap_PropertySubjectToOrAvailableForOperatingLeaseAxis
= good_GroundLeasesMember
For the year ended December 31, 2016 169us-gaap_OperatingLeasesFutureMinimumPaymentsDueInTwoYears
/ us-gaap_PropertyPlantAndEquipmentByTypeAxis
= good_TulsaOkMember
/ us-gaap_PropertySubjectToOrAvailableForOperatingLeaseAxis
= good_GroundLeasesMember
For the year ended December 31, 2017 169us-gaap_OperatingLeasesFutureMinimumPaymentsDueInThreeYears
/ us-gaap_PropertyPlantAndEquipmentByTypeAxis
= good_TulsaOkMember
/ us-gaap_PropertySubjectToOrAvailableForOperatingLeaseAxis
= good_GroundLeasesMember
For the year ended December 31, 2018 169us-gaap_OperatingLeasesFutureMinimumPaymentsDueInFourYears
/ us-gaap_PropertyPlantAndEquipmentByTypeAxis
= good_TulsaOkMember
/ us-gaap_PropertySubjectToOrAvailableForOperatingLeaseAxis
= good_GroundLeasesMember
For the year ended December 31, 2019 169us-gaap_OperatingLeasesFutureMinimumPaymentsDueInFiveYears
/ us-gaap_PropertyPlantAndEquipmentByTypeAxis
= good_TulsaOkMember
/ us-gaap_PropertySubjectToOrAvailableForOperatingLeaseAxis
= good_GroundLeasesMember
For the year ended December 31, 2020 169good_OperatingLeasesFutureMinimumPaymentsDueInSixYears
/ us-gaap_PropertyPlantAndEquipmentByTypeAxis
= good_TulsaOkMember
/ us-gaap_PropertySubjectToOrAvailableForOperatingLeaseAxis
= good_GroundLeasesMember
Thereafter 85good_OperatingLeasesFutureMinimumPaymentsDueInSixYearsAndThereafter
/ us-gaap_PropertyPlantAndEquipmentByTypeAxis
= good_TulsaOkMember
/ us-gaap_PropertySubjectToOrAvailableForOperatingLeaseAxis
= good_GroundLeasesMember
Ground Leases [Member] | Dartmouth, MA [Member]  
Operating Leased Assets [Line Items]  
Lease End Date 2036-05
For the year ended December 31, 2015 174us-gaap_OperatingLeasesFutureMinimumPaymentsRemainderOfFiscalYear
/ us-gaap_PropertyPlantAndEquipmentByTypeAxis
= good_DartmouthMaMember
/ us-gaap_PropertySubjectToOrAvailableForOperatingLeaseAxis
= good_GroundLeasesMember
For the year ended December 31, 2016 174us-gaap_OperatingLeasesFutureMinimumPaymentsDueInTwoYears
/ us-gaap_PropertyPlantAndEquipmentByTypeAxis
= good_DartmouthMaMember
/ us-gaap_PropertySubjectToOrAvailableForOperatingLeaseAxis
= good_GroundLeasesMember
For the year ended December 31, 2017 174us-gaap_OperatingLeasesFutureMinimumPaymentsDueInThreeYears
/ us-gaap_PropertyPlantAndEquipmentByTypeAxis
= good_DartmouthMaMember
/ us-gaap_PropertySubjectToOrAvailableForOperatingLeaseAxis
= good_GroundLeasesMember
For the year ended December 31, 2018 174us-gaap_OperatingLeasesFutureMinimumPaymentsDueInFourYears
/ us-gaap_PropertyPlantAndEquipmentByTypeAxis
= good_DartmouthMaMember
/ us-gaap_PropertySubjectToOrAvailableForOperatingLeaseAxis
= good_GroundLeasesMember
For the year ended December 31, 2019 174us-gaap_OperatingLeasesFutureMinimumPaymentsDueInFiveYears
/ us-gaap_PropertyPlantAndEquipmentByTypeAxis
= good_DartmouthMaMember
/ us-gaap_PropertySubjectToOrAvailableForOperatingLeaseAxis
= good_GroundLeasesMember
For the year ended December 31, 2020 174good_OperatingLeasesFutureMinimumPaymentsDueInSixYears
/ us-gaap_PropertyPlantAndEquipmentByTypeAxis
= good_DartmouthMaMember
/ us-gaap_PropertySubjectToOrAvailableForOperatingLeaseAxis
= good_GroundLeasesMember
Thereafter 3,127good_OperatingLeasesFutureMinimumPaymentsDueInSixYearsAndThereafter
/ us-gaap_PropertyPlantAndEquipmentByTypeAxis
= good_DartmouthMaMember
/ us-gaap_PropertySubjectToOrAvailableForOperatingLeaseAxis
= good_GroundLeasesMember
Ground Leases [Member] | Springfield, MA [Member]  
Operating Leased Assets [Line Items]  
Lease End Date 2030-02
For the year ended December 31, 2015 86us-gaap_OperatingLeasesFutureMinimumPaymentsRemainderOfFiscalYear
/ us-gaap_PropertyPlantAndEquipmentByTypeAxis
= good_SpringfieldMaMember
/ us-gaap_PropertySubjectToOrAvailableForOperatingLeaseAxis
= good_GroundLeasesMember
For the year ended December 31, 2016 86us-gaap_OperatingLeasesFutureMinimumPaymentsDueInTwoYears
/ us-gaap_PropertyPlantAndEquipmentByTypeAxis
= good_SpringfieldMaMember
/ us-gaap_PropertySubjectToOrAvailableForOperatingLeaseAxis
= good_GroundLeasesMember
For the year ended December 31, 2017 89us-gaap_OperatingLeasesFutureMinimumPaymentsDueInThreeYears
/ us-gaap_PropertyPlantAndEquipmentByTypeAxis
= good_SpringfieldMaMember
/ us-gaap_PropertySubjectToOrAvailableForOperatingLeaseAxis
= good_GroundLeasesMember
For the year ended December 31, 2018 90us-gaap_OperatingLeasesFutureMinimumPaymentsDueInFourYears
/ us-gaap_PropertyPlantAndEquipmentByTypeAxis
= good_SpringfieldMaMember
/ us-gaap_PropertySubjectToOrAvailableForOperatingLeaseAxis
= good_GroundLeasesMember
For the year ended December 31, 2019 90us-gaap_OperatingLeasesFutureMinimumPaymentsDueInFiveYears
/ us-gaap_PropertyPlantAndEquipmentByTypeAxis
= good_SpringfieldMaMember
/ us-gaap_PropertySubjectToOrAvailableForOperatingLeaseAxis
= good_GroundLeasesMember
For the year ended December 31, 2020 90good_OperatingLeasesFutureMinimumPaymentsDueInSixYears
/ us-gaap_PropertyPlantAndEquipmentByTypeAxis
= good_SpringfieldMaMember
/ us-gaap_PropertySubjectToOrAvailableForOperatingLeaseAxis
= good_GroundLeasesMember
Thereafter $ 884good_OperatingLeasesFutureMinimumPaymentsDueInSixYearsAndThereafter
/ us-gaap_PropertyPlantAndEquipmentByTypeAxis
= good_SpringfieldMaMember
/ us-gaap_PropertySubjectToOrAvailableForOperatingLeaseAxis
= good_GroundLeasesMember