Basis of Presentation and Significant Accounting Policies - Additional Information (Details) - USD ($) $ in Millions	3 Months Ended	9 Months Ended
Sep. 30, 2020	Sep. 30, 2020
Accounting Policies [Abstract]
Reduction to research and development expense	$ 0.2	$ 0.2

Increase decrease in research and development expense.

No definition available.

Name:	grtx_IncreaseDecreaseInResearchAndDevelopmentExpense
Namespace Prefix:	grtx_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration