Document and Entity Information - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2021
Mar. 07, 2022
Jun. 30, 2021
Cover [Abstract]      
Document Type 10-K    
Amendment Flag false    
Document Period End Date Dec. 31, 2021    
Document Fiscal Year Focus 2021    
Document Fiscal Period Focus FY    
Entity Registrant Name Gritstone bio, Inc.    
Entity Central Index Key 0001656634    
Current Fiscal Year End Date --12-31    
Title of 12(b) Security Common Stock, $0.0001 par value per share    
Trading Symbol GRTS    
Security Exchange Name NASDAQ    
Entity Well-known Seasoned Issuer No    
Entity Current Reporting Status Yes    
Entity Interactive Data Current Yes    
Entity Voluntary Filers No    
ICFR Auditor Attestation Flag false    
Entity Filer Category Accelerated Filer    
Entity Common Stock, Shares Outstanding   72,708,342  
Entity Small Business true    
Entity Emerging Growth Company true    
Entity Ex Transition Period true    
Entity Shell Company false    
Entity Public Float     $ 419.0
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    
Document Annual Report true    
Document Transition Report false    
Documents Incorporated by Reference Portions of the Registrant’s Definitive Proxy Statement relating to the Annual Meeting of Shareholders, scheduled to be held on June 17, 2022, are incorporated by reference into Part III of this Annual Report on Form 10-K where indicated. Such Definitive Proxy Statement will be filed with the Securities and Exchange Commission within 120 days after the fiscal year to which this report relates.    
Auditor Name Ernst & Young LLP    
Auditor Location Redwood City, California    
Auditor Firm ID 42