GTK does not support OpenGL directly. GTK is part of a suit of libraries and provides the main graphical interface such as windows, buttons, menus, tree views, file selectors etc. http://developer.gnome.org/platform-overview/stable/
lists the whole platform of libraries. To use OpenGL you will need to use Clutter http://developer.gnome.org/platform-overview/stable/clutter.html.en
You do not say which version of GTK you are using, so this only applies to GTK 3 and later and GTK 2.8 and later (from what I can see).