With Ruby you can use Tk, which is a mature, cross platform UI toolkit. It is the defacto GUI toolkit for Python and Tcl, and is also available for use with Perl. The most recent versions of Tk make use of native widgets which addresses the primary concern that Tk looks dated.
A language-neutral website devoted to Tk is http://www.tkdocs.com/ which includes examples coded in both Ruby and Tcl.
