Open Use API to receive notifications

Hi dear devs, and thank you for the awesome app.
I would like to develop an iOS app for myself, so I can receive the notifications from my Android phone on my iPod, and reply from my iPod.
But before I begin, I would like to know if this is technically possible.
After going through the API, it seems I can use it to send a notification to my phone but not the opposite (to receive the notification from my phone).
But since the Chrome extension can do that, I wonder if I can do the same with an app.
It seems that Join uses GCM, but I guess I should somehow register as a client my Join app will recognize.
Thanks and sorry if I am asking for something impossible.