You'll have Ruby/GTK, which allows you to use the GTK toolkit under linux. I think that should be working under Windows and Mac Os (as for Gimp, Gaim and so on).
A french magazine post a good beginner article about Ruby/GTK.
Edit : According to main page on the SourceForge project, Ruby-Gnome2 (aka Ruby/GTK) is cross-platform (Windows, Linux, Mac Os).
