I am playing with Google\'s OAuth 2.0 Playground using my own personal Google account, but I cannot seem to recover my Gmail address using the playground.
You'll want to add the https://www.googleapis.com/auth/userinfo.email scope or replace https://www.googleapis.com/oauth2/v2/userinfo with it. If you're using the HTML example they provide, you can list multiple scopes separated by a space.