There is also FxRuby which uses the Fox toolkit, but I haven't looked into it.
Ruby's use of gtk is pretty nice, e.g.
require 'gtk2'
Gtk.init
button = Gtk::Button.new("Hello, world!")
window = Gtk::Window.new
window.signal_connect("destroy") { Gtk.main_quit }
window.border_width = 10
window.add button
window.show_all
Gtk.main
but if you are writing a GUI application, I would still recommend Python over Ruby.