Here you can find the GTK+ Framework ready to download and install on your Mac OS X system. Then you are all set for developing GTK+ applications on your Mac.
RGtk2 is a binding for R to the GTK2 library and dependent libraries. You can use it to construct arbitrarily complex GUI's from R. Your GUI can be integrated with other GTK GUI's such as that of GGobi via rggobi. It works on all three major platforms (Linux, Mac, and Windows).
"Clutter is an open source software library for creating fast, visually rich and animated graphical user interfaces. Clutter uses OpenGL (and optionally OpenGL ES for use on Mobile and embedded platforms) for rendering [...]"