I have used Gtk2 for a while and, while I don't argue it is better or not than Tk or Wx, its integration with Perl certainly makes it worth on point of any disadvantage in front of its competitors. It's so tightly integrated that you can create a model class for some component in Perl and use an instance of that class in a widget that is not aware of Perl at all.

That sums up with Pango (for unicode support), the fact that it is the standard GUI library for the majority of Linux machines and the fact that it is cross-platform as well.