Greetings Ruby gurus

I have recently started using ruby (cool!) and have written a couple of
simple programs with Ruby/Tk interfaces.  They work fine but I have been
unable to access Tk without using a barrow load of global variables, which
goes against the grain.  Ruby/GTK allows encapsulation of the gui within a
class which is what I want but I am having difficulties with gtk2, also a
new area for me. There does not seem to be a ListBox widget, specifically a
scrolling listbox. This is a fundamental type of widget I would have
thought; it is not mentioned in any of the documentation that is available
from standard sources and using strings and grep on gtk2.so returns
nothing for listbox.  Googling does not return anything useful except to
indicate that most other gui toolkits have something of the kind.  I do
not know enough about ruby or GTK to attempt coding a listbox myself so
does anybody know where in the ruby domain such a thing may be found?

Apologies for the fact that this is not a coding question.  If I cannot
find a listbox implementation I shall have to abandon GTK, which I prefer
because I work under GNOME.  wxWidgets installed but resisted all attempts
to run it.