GtkGLArea is an OpenGL widget for GTK+ GUI toolkit.

Just as GTK+ is built on top of GDK, GtkGLArea is built on top of gdkgl which
is basically a wrapper around GLX functions.  The widget itself is very
similar to the GtkDrawingArea widget and adds only three extra functions.
Lower level gdkgl functions make it easy to render on any OpenGL capable
widget; rendering to off-screen pixmaps is also supported.

This project appears to be abandoned in favor of GtkGLExt, but is still
required by a few programs.