How to migrate from GCM to FCM?
This guide will show you how to migrate from use of GCM (Google Cloud Messages) to FCM (Firebase Cloud Messages).
1.) go to https://console.firebase.google.com/
2.) Click -> or Import a google project
3.) Click -> Add Firebase to you Android app
4.) enter your app package name and press -> Add app
5.) download and add file "google-services.json" to you app
6.) Add a lines in your build.gradle files (two different files - be careful)