As of April 10, 2018, Google has deprecated GCM. The GCM server and client APIs are deprecated and will be removed as soon as April 11, 2019. Migrate GCM apps to Firebase Cloud Messaging (FCM), which inherits the reliable and scalable GCM infrastructure, plus many new features.
You can migrate an existing GCM client app on Android to Firebase Cloud Messaging (FCM) using the instructions in this guide. Before getting started, keep in mind: GCM and FCM SDKs cannot co-exist within an application.
GCM tokens retrieved via GoogleCloudMessaging.register() or InstanceID.getToken() will continue to work in FCM without any modification or renewal
With the latest updates of phone systems , user needs and apps it became more than obvious that these last have to be less power and processor consuming. One of the solutions is to make the app dormant and the push notification were created to help on this purpose.
The Concept We want to send a notification to all devices that have your app installed whenever a particular event is triggered. How are other devices going to detect this event? Easy, you need to have a server that will keep listening for this event and as soon as it occurs, this server will notify all the other devices and voila, you can now create notifications! The easiest way to accomplish this is by using Firebase Cloud Messaging(FCM).
Conclusion With the work of firebase company alongside google services this switching from GCM to FCM is very easy and costs more code removing than adding which for a developer is nice and eases you app life promising better notification app messaging service and better management on the back side