AFAIK you will need to add the IP of the sending device/server in google
console as trusted - so no, you usually do not use the client to send push
to other devices. Instead, send the message to your server, which has all
the registered devices to your GCM token, and let the server push the
message to the specific client.
You should read the documentation http://developer.android.com/google/gcm/index.html