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Real Estate Dispositions, Held for Sale, and Impairment Charges - Additional Information (Detail) | 12 Months Ended
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Dec. 31, 2019 USD ($) property | Dec. 31, 2018 USD ($) property | Dec. 31, 2017 USD ($) property
Schedule Of Real Estate Owned Held For Sale [Line Items] |  |  |
Number of impaired assets, held and used | 0 |  |
Number of real estate properties, held for sale | 1 | 1 |
Impairment charge | $ | $ 1,813,000 | $ 0 | $ 6,835,000
Held-for-sale or Disposed of by Sale |  |  |
Schedule Of Real Estate Owned Held For Sale [Line Items] |  |  |
Number of impaired assets, sold |  | 1 | 2
Held-for-sale |  |  |
Schedule Of Real Estate Owned Held For Sale [Line Items] |  |  |
Number of real estate properties, held for sale | 1 | 1 |
Number of impaired assets |  |  | 3

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- Definition Number of real estate properties held for sale. + References No definition available. + Details
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Balance Type: | na
Period Type: | instant

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- Definition Number of Impaired Assets, Held and Used + References No definition available. + Details
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- Definition Number of Impaired Assets Sold + References No definition available. + Details
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Namespace Prefix: | good_
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Balance Type: | na
Period Type: | instant

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- Definition Number of impaired Assets + References No definition available. + Details
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Namespace Prefix: | good_
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Balance Type: | na
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- Definition Schedule of real estate owned held for sale line items. + References No definition available. + Details
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- Definition Amount of write-down of assets recognized in the income statement. Includes, but is not limited to, losses from tangible assets, intangible assets and goodwill. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -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 2: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 360 -SubTopic 10 -Section 45 -Paragraph 4 -URI http://asc.fasb.org/extlink&oid=109226348&loc=d3e2420-110228 + Details
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- Details
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Namespace Prefix: |
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- Details
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