def draw
GL.Viewport(0,0,200,200)
GL.ClearColor(1,1,1,1)
GL.Clear(GL::COLOR_BUFFER_BIT)
GL.Enable(GL::LIGHTING)
GL.Enable(GL::LIGHT0)
GL.Light(GL::LIGHT0, GL::DIFFUSE, [1,0,0,1])
GL.Light(GL::LIGHT0, GL::POSITION, [0.5,0.5,-1,0])
Gdk::GL.draw_sphere(true,0.2,20,20)
GL.Flush
end

だとおもう。