I am using the prepackaged Istio on GKE, which comes with a pre-configured ingress gateway that takes a single SSL certificate.

Is there a way to add additional certificates to Google's standard configuration which will survive reset by their configuration tool and persist through upgrades?

The Istio docs describe how to specify multiple certificates if installing the ingress gateway yourself. I could do this if I configured a separate ingress gateway, but would like to use the default one if I could. Google's docs do not list certificates as a modifiable property.