Like another person, I setup mailgun and it is now working without a problem. I suspect someones post about how discourse was send EHLO localhost.localdomain could be the problem. The G-Suite instructions say it automatically rejects any SMTP session that sends a bogus hostname like that.
Eventually, I will know enough to fix that, but I haven’t a clue so far. It would be nice to know how to change that, in case that is the problem with gmail.