Emmanuele Bassi commented:
I'd say anything that starts with gtk.org/api/2.[0-6]/ needs to redirect to developer.gnome.org/gtk/2.24/, e.g.
www.gtk.org/api/2.6/GtkWidget.html → developer.gnome.org/gtk2/2.24/GtkWidget.html www.gtk.org/api/2.2/GtkButton.html → developer.gnome.org/gtk2/2.24/GtkButton.html ...