Setting up Discourse to perform authentication via Google G-Suite using SAML is quite easy once you figure it out.
Add the discourse-saml plugin to your discourse as documented.
To configure the plugin you need some information from Google G-Suite.
In the admin console add a new SAML application and setup a "Custom application".
This will show you the information you need. The URL mentioned in SSO URL needs to
be used as the DISCOURSE_SAML_TARGET_URL. This is something like https://accounts.google.com/o/saml2/idp?idpid=XXXXX
Now you need the certificate information you put in DISCOURSE_SAML_CERT and
DISCOURSE_SAML_CERT_FINGERPRINT. This is a bit more tricky. The content for DISCOURSE_SAML_CERT