I know that the project is old, but I've personally never heard of much being implemented using GNUStep. I was under the impression that it was an abandoned project, though I never bothered to check their homepage.
[I assumed that AfterStep was implemented with GNUStep, but I just looked at the dependency list, and it doesn't include GNUStep...]
Well if you use Ubuntu, then just search for gnustep, or ".app" or anything that might require GNUStep and you would find a lot of apps.
The problem for me, is the menu system - in theory it really looks cool, that you can detach menus, and leave them as commands (which modern UI's don't do, except maybe Autodesk Maya).
But then I just can't get used to it - give me Window's menus, or Mac OS X ones, or Gnome/KDE and I'm fine - I just can't adjust to the NextStep/OpenStep/GNUStep (okay, my casing is bad here).
[I assumed that AfterStep was implemented with GNUStep, but I just looked at the dependency list, and it doesn't include GNUStep...]