title: Google Authentication API: How to get the user's gmail address
body: <p>I've been studying the Google authentication API (AuthSub)... My question is, how do I get the user's account information (at least their Gmail address) after the authentication has passed?  </p>
  
  <p>Because currently, all I get back from the authentication process is a token granting me access to which ever Google service I have specified in the scope, but there's no easy way to even get the user's login id (Gmail address) as far as I can tell...</p>
  
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  <p>If so, what Google service allows me to access the user's information?</p>