If you are the creator of the GPL software you can do what you like, link Firebase to it, distribute the app and release your code as GPL. You're effectively 'granting yourself' a proprietary license to use your code and a GPL license for everyone else to use it.
However it means nobody else can release their own apps (with notifications) on Android based on your code, or anyone else's GPL code. As soon as they link to Firebase and distribute they will violate the GPL by distributing a binary of someone else's GPL code with a proprietary library attached.
You cannot however use anyone else's GPL code while doing this, because those library authors didn't give you permission to relicense their code. It closes off the entire Play Store to the GPL. This is the same thing that caused the FSF to declare Apple's iTunes store as incompatible with the GPL. Curiously, I think the iTunes TOS clause that disallowed copyleft has since been removed, but I haven't heard anyone more qualified than I offer an opinion on whether you can now publish GPL iOS apps.
However it means nobody else can release their own apps (with notifications) on Android based on your code, or anyone else's GPL code. As soon as they link to Firebase and distribute they will violate the GPL by distributing a binary of someone else's GPL code with a proprietary library attached.