GTK doesn't play much of a role in our GUI. When you're looking at most of the main tabs in Ardour (editor, mixer, recorder window, cue page) almost everything you're looking at is either our own Canvas or custom widgets. Still we do sit on top of GTK+ 2's basic infrastructure and still rely on it for the "big widgets" (file browser, tree/listviews, menus, color dialogs, text entry).