Ok, I managed to find the problem. Posting it here in case someone needs it.
The problem was the Airnotifier server wasn't restarted after Android GCM configuration. Simply pressing "Save" at the end of the configuration screen is not enough to start the android GCM service. At this point the behaviour is (e.g. when sending a broadcast notification for debug):
- Apple notification succesfully delivered
- No GCM notification produced
- Error shown on screen:
The solution is to kill the python instance running Airnotifier and then restart it.
I think this is a wrong behaviour because there is a button for starting/stopping Apple APNs service, but there isn't an equivalent for Google GCM; so I'm going to open an issue at Airnotifier's GitHub.