How to set SSH Key Expiry for the gateway users and force to rotate the public keys?

Configure SSH keys to expire after a specific number of days from GUI. This setting allows you to set an expiration period for SSH keys, and once that period is reached, the keys will become invalid for authentication and the user will later have to login via Password. A value of 0 means that SSH keys will never expire.

       1.  Under Settings > General > Authentication > Set User SSH key lifetime.

                  As an example, Below Screenshot says that the SSH user will expire in 1 day. A value 0 means SSH key will never expire. 

Refer : How to add ssh public key for passwordless authentication in ssh

2.  After a designated period has passed and the key for the specified user has expired, attempting to log in from the backend using key authentication will be unsuccessful. However, the user will still be able to access the system using password authentication. This ensures that security measures are maintained while granting access through an alternative authentication method when key-based authentication is no longer valid.

Following is the expired SSH key, the expiration of which is determined by the date configured in the GUI. Upon adding a key, users can access the associated expiration date by pressing the F2 key.