FCM is the new version of GCM under the Firebase brand.
Benefits.
The new FCM SDK:
1.Simplifies client development. You no longer have to write your own registration or subscription retry logic.
2.Enables a new serverless notifications solution with a web console, Firebase Notifications. With Firebase Notifications, anyone can send notifications to target specific audiences based on Firebase Analytics insights. They can then evaluate notification effectiveness by assessing the built-in Notifications Funnel Analysis in the console.
Refer this link for More info — https://developers.google.com/cloud-messaging/faq
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solved What Are the difference between GCM and FCM?