| Document and Entity Information | Jun. 20, 2021 |
| Cover [Abstract] | |
| Amendment Flag | false |
| Entity Central Index Key | 0001656634 |
| Document Type | 8-K |
| Document Period End Date | Jun. 20, 2021 |
| Entity Registrant Name | Gritstone bio, Inc. |
| Entity Incorporation State Country Code | DE |
| Entity File Number | 001-38663 |
| 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 |
| Written Communications | false |
| Soliciting Material | false |
| Pre Commencement Tender Offer | false |
| Pre Commencement Issuer Tender Offer | false |
| Security 12b Title | Common Stock, par value $0.0001 |
| Trading Symbol | GRTS |
| Security Exchange Name | NASDAQ |
| Entity Emerging Growth Company | true |
| Entity Ex Transition Period | true |
