If the users are unwilling to learn the command-line, they're probably also unwilling to learn an alien widget set, such as Tk. For inexperienced users, I'd stick with an interface they're already going to know: CGI (possibly enhanced with some simple AJAX stuff) if that will do what you need, or platform-native widgets (probably Win32, possibly via Wx for later portability) otherwise.

When people say they want a GUI, they usually mean they want a *familiar* GUI. If you hand them Tk or Xaw, they're probably going to think it's some kind of sick practical joke, and they're not likely to be amused. Gtk would probably annoy them somewhat less, but it's still probably not going to quite be what they really want.