I haven't used GTK+ myself, but I am concerned by comments
that its port to the Windows environment is "mediocre." See,
for example, the otherwise enthusiastic Sun World article at

http://www.sunworld.com/sunworldonline/swol-11-2000/swol-1110-gtk.html

If Ruby doesn't provide an attractive GUI environment for
Windows developers, a very large community of developers
will have been written off.

Richard Schulman