> I'd like to create a simple GUI app that contains some dynamic sketches
> consisting of things like triangles, circles, and rectangles.

I think the answer depends on what you mean by "dynamic" sketches. But any
of the popular GUI toolkits for Ruby (e.g. Ruby/Tk, FXRuby, and Ruby/GTK)
will let you have a blank canvas on which to draw stuff. And all three will
work equally well on Linux.