The quickest way is to add the following code to GCMIntentService.java into the onMessage function. The GCM plugin you are using only receives the GCM message, but you have to trigger notifications on your own. You can also try a cordova plugin for status bar notifications.
protected void onMessage(Context context, Intent intent) {
Log.d(TAG, "onMessage - context: " + context);
// Extract the payload from the message
Bundle extras = intent.getExtras();
if (extras != null) {
String message = extras.getString("message");
String title = extras.getString("title");
Notification notif = new Notification(R.drawable.ic_launcher, message, System.currentTimeMillis() );
notif.flags = Notification.FLAG_AUTO_CANCEL;
notif.defaults |= Notification.DEFAULT_SOUND;
notif.defaults |= Notification.DEFAULT_VIBRATE;
Intent notificationIntent = new Intent(context, SomeActivity.class);
notificationIntent.addFlags(Intent.FLAG_ACTIVITY_SINGLE_TOP);
PendingIntent contentIntent = PendingIntent.getActivity(context, 0, notificationIntent, 0);
notif.setLatestEventInfo(context, "Title of notification", message, contentIntent);
String ns = Context.NOTIFICATION_SERVICE;
NotificationManager mNotificationManager = (NotificationManager) context.getSystemService(ns);
mNotificationManager.notify(1, notif);