I'll offer the little that I know and hopefully a Firebase engineer with provide a more authoritative answer.
There seems to be work underway for a Firebase replacement to GcmNetworkManager. It's Firebase Android JobDispatcher:
The Firebase Android JobDispatcher is a library that provides a
high-level wrapper around job scheduling engines on Android, starting
with the GCM Network Manager.
This replaces the old GCM Network Manager library.
A note in the description states: This library is actively supported by Google engineers.
I haven't used it yet. I'm still using GcmNetworkManager
. My app includes the play-services-gcm
library along with most of the Firebase libraries. That doesn't appear to cause any conflicts. My GCM tasks work as they did before I added Firebase.