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.
Select your GCM Project from the list of existing Google Cloud Project for which you would like to generate a new Server Key(FCM KEY)
Once you open the Project, it will give you the Project overview. Select Settings--> Project Settings.
You will now be able to see an option to add a new Server Key in the ‘Cloud Messaging’ tab.
5. Copy this Server Key.
6. Send this Server Key along with a screenshot of the same page in your Firebase account 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 :)