Update : only today Firebase sent outdated email notifications to GCM.
In 2016, we launched Firebase Cloud Messaging (FCM), the successor to Google Cloud Messaging (GCM). Firebase Cloud Messaging, as the next evolution of GCM, allows you to send notifications and data messages for free on iOS, Android and the Internet. In addition, FCM provides you with new features, such as an easy-to-use notification interface in the Firebase console, so you can easily configure and test notifications to re-engage users.
To devote more time and attention to improving FCM, it was announced today that you should switch to FCM next year. The GCM server and client APIs are out of date and will be removed as soon as April 11, 2019. We recommend that you upgrade sooner rather than later so that you can take advantage of the new FCM features today.
If you have projects that still use the GCM APIs, you need to update the client and server code to use FCM before April 11, 2019. Existing GCM tokens will continue to work with FCM, so you won’t lose the ability to send to your existing users.
At the moment, there is still no official date when the GCM will be officially declared obsolete. As you can see from the frequently asked questions on GCM :
Will GCM be deprecated?
We will continue to support the current version of the GCM SDK for Android and iOS, as we know that many developers today use the GCM SDK to process notifications, and updating the client application takes time.
But all new client-side features will only be added to the FCM SDK in the future. It is highly recommended that you upgrade to the FCM SDK.
Since many developers still use GCM, I am sure that support will continue for quite some time.
However, if you intend to use push notification, I highly recommend continuing with FCM. Keeping migration issues in the future (not so difficult) and using new features available.