https://support.google.com/mail/answer/6594?co=GENIE.Platform%3DDesktop&hl=en#zippy=%2Cactions
First, you have to enable the keyboard shortcut option. Go to Settings > All Settings > Keyboard shortcuts and enable it
| Action | Shortcut |
|---|---|
| Move focus to toolbar | , |
| Select conversation | x |
| Toggle star/rotate among superstars | s |
| Archive | e |
| Delete | # |
| Reply | r |
| Reply in a new window | Shift + r |
| Reply all | a |
| Reply all in a new window | Shift + a |
| Forward | f |
| Forward in a new window | Shift + f |
| Update conversation | Shift + n |
| Archive conversation and go previous/next | ] or [ |
| Undo last action | z |
| Mark as read | Shift + i |
| Mark as unread | Shift + u |
| Mark unread from the selected message | _ |
| Mark as important | + or = |
| Mark as not important | - |
| Snooze. Note: This shortcut isn't available in classic Gmail. | b |
| Expand entire conversation | ; |
| Collapse entire conversation | : |