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Real Estate Dispositions, Held for Sale and Impairment Charges - Components of Income from Real Estate and Related Assets Held for Sale (Detail) $ in Thousands | 3 Months Ended
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Mar. 31, 2020 USD ($) property | Mar. 31, 2019 USD ($)
Real Estate Properties [Line Items] |  |
(Loss) gain on sale of real estate, net | $ (12) | $ 2,952
Assets disposed of by sale |  |
Real Estate Properties [Line Items] |  |
Operating revenue | 0 | 295
Operating expense | 33 | 70
Other expense, net | (12) | (1)
(Expense) income from real estate and related assets sold | (45) | $ 224
(Loss) gain on sale of real estate, net | $ 0 |
Number of properties sold | property | 1 |

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- Definition Disposal Group, Including Discontinued Operation, Other Income (Expense) + References No definition available. + Details
Name: | good_DisposalGroupIncludingDiscontinuedOperationOtherIncomeExpense
Namespace Prefix: | good_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

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- Definition Income loss from real estate and related assets held for sale. + References No definition available. + Details
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Namespace Prefix: | good_
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- Definition Number of Non-Core Properties Sold + References No definition available. + Details
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Data Type: | xbrli:integerItemType
Balance Type: | na
Period Type: | duration

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- Definition Amount of operating expense attributable to disposal group, including, but not limited to, discontinued operation. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 205 -SubTopic 20 -Section 50 -Paragraph 5B -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=109222650&loc=SL51721673-107760 + Details
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- Definition Amount of revenue attributable to disposal group, including, but not limited to, discontinued operation. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 205 -SubTopic 20 -Section 50 -Paragraph 5B -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=109222650&loc=SL51721673-107760 + Details
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Balance Type: | credit
Period Type: | duration

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- Definition The difference between the carrying value and the sale price of real estate or properties that were intended to be sold or held for capital appreciation or rental income. This element refers to the gain (loss) included in earnings and not to the cash proceeds of the sale. This element is a noncash adjustment to net income when calculating net cash generated by operating activities using the indirect method. + References Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef -Publisher FASB -Name Accounting Standards Codification -Topic 360 -SubTopic 10 -Section 50 -Paragraph 3 -Subparagraph (c) -URI http://asc.fasb.org/extlink&oid=109226691&loc=d3e2941-110230 Reference 2: http://www.xbrl.org/2009/role/commonPracticeRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 45 -Paragraph 28 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=118932676&loc=d3e3602-108585 Reference 3: http://www.xbrl.org/2009/role/commonPracticeRef -Publisher FASB -Name Accounting Standards Codification -Topic 360 -SubTopic 10 -Section 45 -Paragraph 5 -URI http://asc.fasb.org/extlink&oid=109226348&loc=d3e2443-110228 + Details
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- Definition Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table. + References No definition available. + Details
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- Details
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