Schedule III - Real Estate and Accumulated Depreciation and Amortization Part 22 (Parenthetical) (Detail)	Dec. 31, 2017 USD ($)
Real Estate And Accumulated Depreciation Disclosure [Abstract]
Aggregate cost for federal income tax	$ 967,355,000

Real estate and accumulated depreciation carrying amount of land and buildings and improvements for tax purposes.

No definition available.

Name:	gty_RealEstateAndAccumulatedDepreciationCarryingAmountOfLandAndBuildingsAndImprovementsForTaxPurposes
Namespace Prefix:	gty_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant