Open Gmail app > Settings > click on your email id > enable notifications and notification sounds. Close Gmail app and restart your phone. See below the screenshots of how it is configured for me. Check if this helps resolve the issue.
I have spent a week trying the workaround as suggested with the developers. I know my way round a Samsung phone, but Gmail notifications just don't happen on my s21. S10 ,pc, and laptop all get notifications. So Gmail is disabled and I will use the Samsung email app. Of which I get notifications. Thanks for the help, but I've had it with the Gmail app.