How to use GCM in OpenHab android app

Hello, I have installed openhab android app on my android phone, I have also configured sensors such as flood alarm . But I am not getting notification when alarm gets triggered. How Can I do it.

Write a rule and use the notification actions:

unfortunately I don’t understand this GCM stuff. Do I need to create an account somewhere? I see the GCM credentials in my local openhab-cloud config, but I don’t know what to enter.

thanks in advance