Still uncrear ... :-(
So, if we would have:
1) Game client Android app
2) Game server app with limited players qty (i'm planning Android one server and B4J for PC, i guess, network part may be the same code module)
So, where\when should be a single common Google project ID ? And other communication ids.....if planned:
1) when a new game server is started - it must send its address to the shared list (on what server ?)
2) Game clients need to get the servers list firstly, or at updating the game servers list
3) then connect to a chosen server for playing.
And how GCM can help here, if say 20 game servers, 640 gamers (clients) on them, and data exchange the server-client maybe should be 3 times per a second ?
UDP (as usually) here is impossible, i think, as no ensurance in input mobile connections possibility, "white"-IPs for servers.