No authenticated users in Firebase Console

[netotz]

I've succesfully implemented Google sign-in thanks to this NuGet. The problem is when I want to modify the Firebase project rules so only authenticated users can access to the Cloud Firestore database. In my Firebase console appears that the project doesn't have any user:

But in the app I'm using my Google account with no problem:

How can I authenticate Google signed-in accounts so they can appear in Users, so I can modify the rules of the Cloud Firestore? These are my Firebase rules:

rules_version = '2';
service cloud.firestore {
  match /databases/{database}/documents {
    match /{document=**} {
        allow read, write: if request.auth != null
    }
  }
}

Immediately after I sign in with Google, I get Plugin.CloudFirestore.CloudFirestoreException: 'PERMISSION_DENIED: Missing or insufficient permissions.'.

However, if I set the rules to be just true, I can use my Google account with no problems, but for what the exception states, it isn't authenticated (?).