Ruby/GTK2 is a library that allows Ruby applications to use GTK+ 2.x library. GTK+ is open source and is released under the GNU LGPL licence, so it may be used freely for commercial applications. Ruby/GTK2 is available on Linux, Windows, Mac OS.
GTK+ has a rich set of widgets from the basic ones like labels, buttons to the advanced like tree controls etc.
The following URLs will be useful: