Simplest possible registration handler for OpenID Connect login from Google.

GoogleInternalRegHandler.cls

global class GoogleInternalRegHandler implements Auth.RegistrationHandler{
    global User createUser(Id portalId, Auth.UserData data){
        User u = [SELECT Id FROM user WHERE GoogleID__c =: data.identifier];
        return u;
    }
    
    global void updateUser(Id userId, Id portalId, Auth.UserData data){
    }
}

Helped a lot in my use case.
I was quite confused since I couldn't get why I had to create a new user to make it work... but your snipped just showed my the way ;)
Thanks!