Google has deprecated GCM, integrated Cloud Messaging with Firebase, and introduced many improvements.
Here are the steps to help you migrate from GCM to FCM :
Login to your Firebase Console using the Google account on which the GCM Project was created. Click on Add Project
Select your GCM Project from the list of existing Google Cloud Projects.
Click on Add Firebase
Your project will now be migrated to FCM. Click on the Gear icon next to Project Overview and then click on Project Settings
You will now be able to see a new Server Key in the ‘Cloud Messaging’ tab. Copy this Server Key.
Send it in an email to firstname.lastname@example.org asking us to update the Server Key on your dashboard. Our team will take care of the rest :)