At attempt to stitch Google's OAuth into the SMTP module. I've forked SMTP's 8.x-1.x and added in required code to make Google OAuth work.

Setup with Google OAuth is quite tricky. You can find instructions on the setup page at /admin/config/system/smtp_oauth_setup . This page not only walks through the Google Developer Console setup, but is required to pass the ClientID and Client Secret and retrieve the Refresh Token from Google.