I have developed a push notification application in Android
from this tutorial:push notification in android app. The register button is displayed when I run the
personally suggest you that instead of GCM there is also other library named Parse for PushNotification, it works same as Google Cloud Messaging but it is so so so so much easy then GCM
You have to just download JAR file and simple two-three line of code for PUSH-NOTIFICATION
to learn use this site https://parse.com/tutorials/android-push-notifications
Even you don't have to use PHP or any kind of server side code it provide you facility
look i will give u demo
Parse.initialize(this, "YOUR_APP_ID", "YOUR_CLIENT_KEY");
PushService.setDefaultPushCallback(this, YourDefaultActivity.class);
from above code is enough for receiving push notification
if you want to send notification they provide nice UI look the picture of UI they provide