Introduction to development for notification mail sending with Gmail

Google Workspace isn’t designed for transactional emails. You’ll need to use something else.

If you want to have your forum receive mail, see Configure direct-delivery incoming email for self-hosted sites with Mail-Receiver

Typically, you’d configure some transactional mail service to send from noreply@forum.example.com and the mail receiver would receive mail sent to it and hand it off to discourse.