Setting up Google Play Authentication will first require downloading and importing the Google Play Game Services plugin for Unity. This plugin, including the steps required to set up your app on Google Play and link to your Unity build, can be found here.
 
Next, you will need to acquire a serverAuthCode from Google Play and pass it to GooglePlayConnectRequest. This will link the GameSparks player to their Google Play account.

If “NOTAUTHENTICATED” is returned, please check the following:

• There is a Web App linked to your Google Play game service, and that you have setup your Google integration in the GameSparks portal using the Client ID & Client Secret for this application.
• If you are using a local web server, redirectUri is used. More Info.
• Ensure the Uri you send with the request matches that which you can find inside your linked Web App. Navigating to the API Console  →  clicking Edit on the credential for your Web App  →  Ensure the Uri matches 
• Ensure the Google Play user connecting to GameSparks is authorized as a Test User and Enable Alpha / Beta testing  allowing Users to generate a serverAuthCode.