Well, I'm not a pro like Carlo seems to be, but meaby this can help you in this proyect or another:

 https://www.ruby-toolbox.com/categories/GUI_Frameworks

 RubyMine for me is the best GUI that exit for Ruby(I've tested a fews), I repeat: my opinion, it's not free.

 I'm not sure but I've seen a software like the one you like to develop(I guess...):

 http://www.confreaks.com/videos/848-goruco2008-archaeopteryx-a-ruby-midi-generator

 It's an old project, it never seen the light: https://github.com/gilesbowkett/archaeopteryx
 ...but meaby can be an inspiration

 Kind regards. Damin.