Well if your using C++ and Gtkmm then you would want to use gtkglextmm there are examples here http://gtkglext.sourceforge.net/referen ... mples.html
and here is the api docs http://gtkglext.sourceforge.net/referen ... index.html
As far as not being developed anymore.
GtkGLExt has new maintainers! More information will be available soon.
On a philosophical note, GtkGLExt really shouldn't exist at this point. Given the strong ties between GTK -> Cairo and Cairo -> OpenGL, it is surprising that OpenGL support hasn't been integrated into GTK already. See http://bugzilla.gnome.org/show_bug.cgi?id=119189
for a conversation on this topic.