GCM Client App on Android – https://developers.google.com/cloud-messaging/android/client
android sample – https://github.com/googlesamples/google-services/tree/master/android/gcm/app/src/main/java/gcm/play/android/samples/com/gcmquickstart
other devices
Set up a Client App on iOS
https://developers.google.com/cloud-messaging/ios/client