You can use the HTTP interface for GCM servers as described in this Android developers page .
The easiest way to do this is to download Google Cloud Messaging for the Android library using the Android SDK Manager, which will provide you with a bank that can handle the complexity of setting up requests and responses to GCM.
At the bottom of this page, there is an example Java push notification application in App Engine that you can use to familiarize yourself with sending push notifications and then adapt to your needs.
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