Did you start getting this message out of the blue from an iOS device like an iPhone that was working fine just a few hours ago, and you haven’t made any changes?
Before you use the nuclear option of removing and re-creating the account, try this suggestion from faridbe on the Apple Support Forums. Sign into gmail using a web browser. Go to settings, POP/IMAP, then disable IMAP, save changes, then enable IMAP, save changes again.
Then try accessing mail on your device again – perhaps it will work.