But what I found on the internet is using an external webserver, able to run PHP (or Python) scripts and using a MySQL server to store registration IDs.
So far so good, but why do I need to setup an external server with MySQL and PHP just to send messages to the Android devices. The ESP8266 has server capabilities build-in and has enough storage to save registration IDs in the Flash memory.
What I wanted is:
As I could not find any tutorial or example on the Internet how to achieve this, I started to write the code by myself.
And here is the result:
ESP8266 - Google Cloud Messaging without external server
Of course the code is open source and stored in my Github repository
Hope it helps someone.