Firebase Cloud Messaging (commonly referred to as FCM), formerly known as Google Cloud Messaging (GCM), is a cross-platform solution for messages and notifications for Android, iOS, and web applications, which currently can be used at no cost.
The service is provided by Firebase, a company owned by Google. On 21 October 2014, Firebase announced it had been acquired by Google for an undisclosed amount. The official Google Cloud Messaging website points to Firebase Cloud Messaging (FCM) as the new version of GCM.
For portability of web push subscribers it is recommended that all websites use their own FCM key, especially if using third party tools.
References
Firebase Cloud Messaging Wikipedia(Text) CC BY-SA