Document and Entity Information - shares	3 Months Ended
	Mar. 31, 2023	May 08, 2023
Cover [Abstract]
Document Type	10-Q
Amendment Flag	false
Document Period End Date	Mar. 31, 2023
Document Fiscal Year Focus	2023
Document Fiscal Period Focus	Q1
Document Quarterly Report	true
Document Transition Report	false
Entity Registrant Name	Gritstone bio, Inc.
Entity Central Index Key	0001656634
Current Fiscal Year End Date	--12-31
Entity Filer Category	Non-accelerated Filer
Entity Small Business	true
Entity Emerging Growth Company	true
Entity Ex Transition Period	true
Entity Current Reporting Status	Yes
Entity Interactive Data Current	Yes
Entity File Number	001-38663
Entity Incorporation, State or Country Code	DE
Entity Tax Identification Number	47-4859534
Entity Address, Address Line One	5959 Horton Street
Entity Address, Address Line Two	Suite 300
Entity Address, City or Town	Emeryville
Entity Address, State or Province	CA
Entity Address, Postal Zip Code	94608
City Area Code	510
Local Phone Number	871-6100
Title of 12(b) Security	Common Stock, $0.0001 par value per share
Trading Symbol	GRTS
Security Exchange Name	NASDAQ
Entity Shell Company	false
Entity Common Stock, Shares Outstanding		88,913,562