Signal has sworn by GCM the entire time they've been running on Android, it took a lot of whining from the community to get them to allow anything else.
The reason to do this is security principle: Don't Stand Out.
If Bob uses Bob's personal notification system for Signal, a passive observer can see that Bob was notified by James. If everybody uses Huge Corp notifications the passive observer can only tell that James was notifying somebody about something and that Bob was notified about something.
