Hello Tim,
I can confirm that Google, as an Email System Provider, is available in our 9 Series GA
For your reference
https://docs.whmcs.com/9-0/system/email/mail-providers/
https://docs.whmcs.com/9-0/system/email/email-tutorials/use-google-for-system-mail/
Basically, we use OAuth 2.0 in our product and have removed basic auth in line with best practices for Google and similar providers on this.
As you appreciate, we work on industry standards.