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Real Estate Dispositions, Held for Sale and Impairment Charges - Additional Information (Detail) - property | Mar. 31, 2021 | Dec. 31, 2020
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Boston Heights, Ohio; Rancho Cordova, California; Champaign, Illinois |  |
Real Estate Properties [Line Items] |  |
Number of real estate properties, held for sale |  | 3
Rancho Cordova, California and Champaign, Illinois |  |
Real Estate Properties [Line Items] |  |
Number of properties sold | 2 |

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- Definition Number of real estate properties held for sale. + References No definition available. + Details
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- Definition Number of Real Estate Properties Sold + References No definition available. + Details
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Namespace Prefix: | good_
Data Type: | xbrli:integerItemType
Balance Type: | na
Period Type: | instant

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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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Namespace Prefix: | us-gaap_
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Balance Type: | na
Period Type: | duration

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- Details
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Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

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- Details
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Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |