In addition to the documentation differences, another advantage of Tk over Gtk is it's ease of installation, and cross-platform compatibility. Tk installs easily on most platforms, and is especially compatible with Windows.

Gtk can be a pain to get going. The latest version requires a whole slew of the latest libraries be installed, with a properly configured pkgconfig. This is a real common problem for new users to linux....how do I upgrade my gtk2 libs. Gtk2 also poses some difficulty getting installed on Windows.

On the other hand, the GTK2-perl team have done a great job of getting the perl port going. The perl-gtk2 widget demo almost works perfectly. :-)