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Debt - Summary of Scheduled Debt Maturities, Including Balloon Payments (Parenthetical) (Detail) |  | 6 Months Ended
--- | --- | ---
Jun. 02, 2015 | Jun. 30, 2018
Debt Instrument [Line Items] |  |
Revolving facility optional extension period | 1 year |
Revolving Facility [Member] |  |
Debt Instrument [Line Items] |  |
Credit facility agreement, maturity date |  | Mar. 23, 2022
Revolving facility optional extension period |  | 1 year
Credit facility agreement, optional extended maturity date |  | Mar. 31, 2023

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- Definition Line of credit facility optional extended maturity date. + References No definition available. + Details
Name: | gty_LineOfCreditFacilityOptionalExtendedMaturityDate
Namespace Prefix: | gty_
Data Type: | xbrli:dateItemType
Balance Type: | na
Period Type: | duration

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- Definition The length of time the Revolving Facility may be extended. + References No definition available. + Details
Name: | gty_RevolvingFacilityOptionalExtensionPeriod
Namespace Prefix: | gty_
Data Type: | xbrli:durationItemType
Balance Type: | na
Period Type: | duration

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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
Name: | us-gaap_DebtInstrumentLineItems
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

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- Definition Date the credit facility terminates, in CCYY-MM-DD format. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02.19(b),22(b)) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 + Details
Name: | us-gaap_LineOfCreditFacilityExpirationDate1
Namespace Prefix: | us-gaap_
Data Type: | xbrli:dateItemType
Balance Type: | na
Period Type: | duration

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- Details
Name: | us-gaap_CreditFacilityAxis=us-gaap_RevolvingCreditFacilityMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |