As far as I can tell (I now have the API access, so I might test later), you only need to provide additional authentication credentials to Certbot. The private

As far as I can tell (I now have the API access, so I might test later), you only need to provide additional authentication credentials to Certbot. The private key stays on your server. The CA server _might_ reject requests that do not come from GCP, or attempt to obtain one for a domain name not setup with GCP.