Gotify works with a server.
At this moment there is no public server (open to registration) but you can ask on the matrix chat (or IRC) if someone wants to accept you on her/his server or self-host it.
NoProvider2Push needs a static IP to work, which is reachable with a VPN or public meshing network.
Firebase Cloud Message is Google’s push notification service. You won’t need anything else to use it.