Basis of Presentation and Significant Accounting Policies - Additional Information (Details) $ in Millions	6 Months Ended
Jun. 30, 2022 USD ($)
Accounting Policies [Abstract]
Reduction to research and development expense	$ 0.3
Funding available under the grant	$ 1.1

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