| Document And Entity Information | Nov. 08, 2023 |
| Cover [Abstract] | |
| Document Type | 8-K |
| Amendment Flag | false |
| Document Period End Date | Nov. 08, 2023 |
| Entity Registrant Name | Gritstone bio, Inc. |
| Entity Central Index Key | 0001656634 |
| Entity Emerging Growth Company | true |
| Securities Act 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, 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 |
| Entity Ex Transition Period | true |
| Title of 12(b) Security | Common Stock, $0.0001 par value per share |
| Trading Symbol | GRTS |
| Security Exchange Name | NASDAQ |
