The Google Sign-In API gives OAuth2 JSON Web Tokens (JWT) as response data upon user sign-in. A necessary step for a service provider to trust such a token is validatation. Accepting the token without validation would allow a malicious client to simply assert itself in your system. Google provides libraries in several languages (https://goo.gl/jkzS18) to accomplish this, as well as an API endpoint that can outsource the task to Google's own servers (thereby introducing an additional network round trip into every authentication step), but a Ruby implementation is missing. This gem fills that gap.