Google Cloud Messaging (GCM) with local device groups on Android gives HTTP Error code 401

Found the trick: you are using a google account to take the id_token, you need to use EXACTLY the email as notification_key_name. So if you are using [email protected], you need to use this address as notification_key_name.

Here's the recap, based on the correct answer by greywolf82. The correct code should follow these principles (error handling etc. has been stripped):

///////////////////////////////////////////////////////////////////////////
// Working example on how to create a locally (client-side) managed      //
// device group for Google Cloud Messaging.                              //
//                                                                       //
// Thanks to greywolf82 for adding the final piece.                      //
// Please vote on his answer. Thank you!                                 //
///////////////////////////////////////////////////////////////////////////

// Get token:
String account = ACCOUNT_NAME; // E.g. "[email protected]"
String scope = "audience:server:client_id:" + WEB_APPLICATION_CLIENT_ID;
String idToken = GoogleAuthUtil.getToken(context, account, scope);

// Get registration id:
InstanceID instanceID = InstanceID.getInstance(this);
String registration_id = instanceID.getToken(
        getString(R.string.gcm_defaultSenderId),
        GoogleCloudMessaging.INSTANCE_ID_SCOPE, null);

// Set up HTTP connection:
URL url = new URL("https://android.googleapis.com/gcm/googlenotification");
HttpURLConnection connection = (HttpURLConnection) url.openConnection();
connection.setRequestMethod("POST");
connection.setDoOutput(true);
connection.setDoInput(true);
connection.setRequestProperty("project_id", NUMERICAL_PROJECT_ID);
connection.setRequestProperty("Content-Type", "application/json");
connection.setRequestProperty("Accept", "application/json");
connection.connect();

JSONObject requestBody = new JSONObject();
requestBody.put("operation", "add");
requestBody.put("notification_key_name", ACCOUNT_NAME); // You *must* use the email!
requestBody.put("registration_ids",
    new JSONArray(Arrays.asList(new String[]{registration_id})));
requestBody.put("id_token", idToken);

// Submit request body
OutputStream os = connection.getOutputStream();
os.write(requestBody.toString().getBytes("UTF-8"));
os.close();

// connection.getResponseCode() is now 200  :-)
// Now read the server response contents from connection.getInputStream()

The submitted JSON to https://android.googleapis.com/gcm/notification looks something like this:

{
  "operation": "add",
  "notification_key_name": "[email protected]",
  "registration_ids": ["very long string here"],
  "id_token": "another very long string"
}

The response contents is:

{
  "notification_key": "your notification key is here --- voilá!"
}