Hi, @gibbon27, are you heading to Settings > Accounts and backup > Manage accounts > Add account > Email to set up your account? What happens when you try to set this up?
I recommend clearing the cache of the Gmail app to see if this resolves your notification issue, @pete007, you won't lose any personal data. Head to Settings > Apps > Gmail > Storage > Clear cache to do this.
@pete007 Ah, I see. It's good to hear you reached out to the developers. It may be a temporary app issue, I recommend keeping an eye on the Play Store for any app updates for Gmail. I'm sure the issue will be sorted out promptly, so you can continue using it on your S21.