If deleting and re-adding your Gmail account doesn't help, check your calendars and see what your subscribed calendars are. I had that popup on all my iOS devices for a long time. I removed all my email accounts and added them back. I reset my settings and nothing worked. And then I looked at my calendar and saw that it was the server for a calendar that I had subscribed to a couple of years ago for an event. Something must have changed and it was showing up as invalid. I removed that calendar subscription and that fixed my issue. if your iOS calendar is subscribed to your Gmail account, that may be part of your problem. https://www.wikicue.com/how-to-fix-imapgmailcom-cannot-be-verified/