To use the Google Play plugin, you must first wait for it to finish loading. Do not attempt to use any features of the plugin until On loaded triggers, or Is loaded is true.
Next the Google Play plugin will automatically try to sign the user in. If they have successfully signed in before, On signed in will trigger soon after loading. If they have never signed in before, On auto-sign in failed will trigger; in this event you should show a button that allows the user to sign in. When they click this button, use the Sign in action; if it succeeds, On signed in will be triggered.
Once the user is signed in, you can make use of the other plugin features such as requesting leaderboards, submitting high-scores, and unlocking achievements."