As of May 2021, GKE Autopilot has no support for 3rd party webhooks. Without webhooks, many Kubernetes plugins such as cert-manager cannot operate correctly. Cert-manager uses a custom mutating admission webhook to manage certificates, which is immutable on GKE Autopilot.

To add/manage SSL certificates for Autopilot clusters, you should first start with this official GCP doc Google-managed SSL certificates.

You can configure Google-managed SSL certificates using a ManagedCertificate custom resource, which is available in different API versions, depending on your GKE cluster version. It's recommended that you use a newer API version.

• ManagedCertificate v1beta2 API is available in GKE cluster versions 1.15 and later.
• ManagedCertificate v1 API is available in GKE cluster versions 1.17.9-gke.6300 and later.

Note: Google-managed SSL certificates aren't currently supported for internal HTTPS load balancers. For internal HTTPS load balancers, use self-managed SSL certificates instead. This feature is only available for Ingress for External HTTP(S) Load Balancing, can read more here.

To configure a Google-managed SSL certificate and associate it with an Ingress, follow the two basic steps first:

• Create a ManagedCertificate object in the same namespace as the Ingress.
• Associate the ManagedCertificate object to an Ingress by adding an annotation networking.gke.io/managed-certificates to the Ingress. This annotation is a comma-separated list of ManagedCertificate resources, cert1,cert2,cert3 for example. Which is mentioned in detail here.

You have to follow some prerequisites:

• You must own the domain name (Google Domains or another registrar).
• Your "kubernetes.io/ingress.class" must be "gce".
• Create a reserved (static) external IP address. If you do not reserve an address, it may change, requiring you to reconfigure your domain's DNS records.

For setting up a Google-managed certificate, go through the sample ManagedCertificate manifest.