Why isn't gmail listed as a recent app when I long-press the Home button?
If you haven't opened the application in a while, it will naturally disappear from the list of recent apps.  Try opening it with the widget and then going back to the home screen.  If gmail is once again under you list of recent apps, you just haven't opened it in a while.  If it doesn't appear at all, then the widget you are using doesn't add gmail as a recent app.