Why isn't gmail listed as a recent app when I long-press the Home button? This would be helpful at times... I usually just end up pressing home and selecting the gmail widget from there but I've gotten used to long-pressing home to get to recent apps.

Could it be due to the fact that I get to email using the widget? Does it work with the native email client or other email apps?

Thanks in advance...
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.