I suggest the official email troubleshooting howto.
Also if you read that topic you’d see
Not entirely sure what you’re asking – you mean in the context of that error? Yes, the G Suite (neé Google Apps) domain is custom, not @gmail.com.
So if your SMTP_ADDRESS is gmail.com that already seems wrong. You should be using your actual domain there, as configured in G suite.
Or per the above, smtp-relay.gmail.com maybe?