Like some suggested, I used RemoteViews
to customize my notification, as following:
final RemoteViews views = new RemoteViews(getPackageName(), R.layout.notification_new_message);
views.setTextViewText(R.id.title, title);
Notification notification = new Notification();
notification.contentView = views;
notification.icon = R.drawable.push_icon;
notification.contentIntent = contentIntent;
manager.notify(Config.NTF_ID_GCM, notification);
Where I set inside my notification_new_message.xml
the desirable look of my notification.