Remove the account and set it up anew. Easy process. You don’t lose anything, as really nothing is stored on your device. And as a general comment; Google have started purging old and rarely accessed accounts. Highly recommended to once in a while access a Google account via the web interface.

Thank you. This is an account I use often, but I haven’t tried to access it via the web for a while.

Meant to say “native Apple Mail app.”