Google Cloud Platform offers Google Managed Certificates for External Load Balancers and GKE Ingress but does not currently support the ability to manage wildcard certs. If you are looking to have Google manage your *.example.com certificate then you are out of luck until the feature becomes available.
This guide outlines how to use cert-manager on GKE to automatically provision a wildcard certificate when your Ingress resource gets created. Cert-manager will perform domain verification, request the certificate from Let’s Encrypt, and handle auto-renewing your certificate before the 90 day expiration date.
I do Cloud architecture, Kubernetes, Service Mesh, CI/CD and other cloud native things. Customer Engineer at Google Cloud. Opinions stated here are my own.