There several ways in which to provide a graphical interface for the RubyLanguage. 

* RubyTk
* Ruby/GTK
* FX/Ruby (FOX)

The above are described in some detail in Chapter 6 of TheRubyWay. In addition, there are

* JavaSwing or JavaSwt when using JayRuby
* RubyShoes, which is new in the field

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See also 

* http://wiki.rubygarden.org/Ruby/page/show/ComparingGuiToolkits for a comparison
* ThenDontCallMainLoop which has an example using Ruby/GTK.
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CategoryRuby