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.