Starting from Sept. 2016 new server keys for Google Cloud Messaging can only be created in the Firebase Console using the Cloud Messaging tab of the Settings panel.
Existing Android projects may encounter a 401 error when sending Push Notifications (https://developers.google.com/cloud-messaging/http-server-ref#error-codes). To resolve this error, the existing project need to be imported into Google Firebase in order to generate a new server key. The new server key then needs to be updated for your App ID on the CommzGate MACH portal. (https://app.themach.com)
Your existing Android projects can be imported in the Firebase console without affecting your existing configuration. After you have imported the existing Google project in your Firebase Console, go to "Overview > Settings" (the Cog icon as per image below).
In Cloud Messaging, you'll see the new server key generated. The old GCM key would be displayed as the legacy key.
Log in to your MACH account (https://app.themach.com) and update your app details with this new server key.