Google is deprecating GCM so, it would be a good idea switching to FCM.
Google Cloud Messaging page:
As of April 10, 2018, Google has deprecated GCM. The GCM server and client APIs are deprecated and will be removed as soon as May 29, 2019. Migrate GCM apps to Firebase Cloud Messaging (FCM), which inherits the reliable and scalable GCM infrastructure, plus many new features. See the migration guide to learn more.
Google is deprecating GCM so, it would be a good idea switching to FCM.
Google Cloud Messaging page: