title: Google App Engine authentication options
body: <p>I'm looking to put an app on Google Cloud (basically going to convert a visual basic project into Javascript and then upload it.) I want to start a subsciption service for a cloud-based application and it's not going to be gargantuan by any stretch of the imagination. Very small (read: not tons of columns or huge amounts of data) and just want to give users an option. </p>
  
  <p>I signed up today and noticed there were 3 options for authentication. However, I don't see alternatives (example: authentication based on a confirmed paid customer account).</p>
  
  <p>Am I to assume that there's no other way to limit access for a Google App Engine app (for proprietary reasons)? </p>