No special setup is required for Google Cloud Messaging. All that is required is that the setting Third-party push notification is enabled in the Central Services setup panel. See Central Services.
You can verify that an Android device is connected in the following way:
Install the DME client for Android on an Android device, and synchronize.
Verify that the device is shown in the Devices tab in DME.
Make sure that Third-party push notification is On in the Preferences > Adaptive push section of device settings (for the device, for the group, or by default). See Settings.
Synchronize the device (which cannot be rooted!) again. A full system information sync will provide a new property called GCM Device Token (visible in Devices > click the Android device > Information), which is necessary for Google Cloud Messaging to work.