As of April 10, 2018, Google has deprecated the Google Cloud Messaging (GCM) service. The GCM server and client APIs are deprecated and will be removed as of April 11, 2019. Migrate GCM apps to Firebase Cloud Messaging (FCM), which inherits the reliable and scalable GCM infrastructure, plus many new features.
The Mattermost Push Notification Service version 5.8+ uses FCM to send Push notifications for Android.
Upgrade your Mattermost Push Notification service to version 5.8 or above
Go to the Firebase Console and select the project you’ve created. Once in the dashboard, go to the project settings and select CLOUD MESSAGING.
Look for the value of the Server Key and copy it.
Open the mattermost-push-proxy.json file in the mattermost-push-proxy/config directory and replace the value for the “AndroidApiKey” setting with the Server Key you just copied.
Restart your Mattermost Push Notification Service, and notifications should be sent using FCM.
$ sudo service mattermost-push-proxy restart
Test push notifications are being delivered as expected.
Make sure your firewall allows connections to the Firebase servers fcm.googleapis.com/fcm and fcm-xmpp.googleapis.com on ports:
80 TCP HTTP client connection
443 TCP HTTP client connection over SSL
5222 TCP XMPP client connection
5223 TCP XMPP client connection over SSL