Hey Kennedy,
Thanks for the feedback. It would indeed be great if Firebase had a closer integration with Google Cloud Messaging. If we ever release something like that, you can rest assured we'll communicate it widely and loudly. :-)
In the meantime setting up a small (Heroku, GCE/GKE or other) server/VM would indeed be the way to go. Keep in mind that you can use the Firebase Database as the intermediary between the app and this server. So you app pushes the "notification requests" to the database, the server process listens for those requests, calls GCM for each and then removes the request from the database. Such a server process would be an example of
Pattern 2: Firebase-powered app with server code.
puf