They do offer client-side encryption for enterprise, but it uses S/MIME rather than PGP.
https://support.google.com/a/answer/10741897
https://security.googleblog.com/2023/06/gmail-client-side-en...
https://support.google.com/a/answer/10741897
https://security.googleblog.com/2023/06/gmail-client-side-en...