From what I have read it depends what platform or flavour of Linux your
app is targetted at.
GTk is great for Gnome
Qt is great for KDE.
http://members.chello.nl/~k.vangelder/ruby/learntk/
There’s also a few others out there to look at like Tk, wxRuby and
FXRuby… Check out this site I just found:
http://wiki.rubygarden.org/Ruby/page/show/ComparingGuiToolkits/TakeTwo
http://wiki.rubygarden.org/Ruby/page/show/DoYourRubyGUIResearch
Hope this is some help!