Hello, @backtrack23 - welcome to the Auth0 Community!
This is happening because the Resource Owner Password Grant can only be executed against connections where the authorization server (in this case, Auth0) has the username and password. For Google connections, we actually federate into Google - we redirect to Google so that they can do the verification, and we never handle the username and password.
I very recently wrote an answer related to this, and I think it can provide an answer to your questions:
Let me know what you think!