To update on this question for anyone that comes after, recently there is a feature in Keycloak that allows to authenticate with the Google token on Keycloak API. It's called Token Exchange.

This is currently a preview feature, so it needs to be enabled by passing some feature flags --features=token-exchange,admin-fine-grained-authz

You can read more about it in this issue in Github, where everything is explained in more detail.

Then just enable token exchange on Keycloak and send the token to the keycloak endpoint /realms/{realm-name}/protocol/openid-connect/token