Unless things changed a lot in the last ~three or so years, Non-Linux platforms like Mac and Windows aren't really in the focus of Gtk+. In the past ~five years a large number of cross-plattform Gtk+ applications redeveloped their UIs in "Not-Gtk+" (usually Qt). Wireshark, Subsurface, Openshot and LXDE come to mind, these are all pretty large, UI-centric projects, so replacing the toolkit is a lot of work and wouldn't be something you'd do just for shits and giggles.

If you really, really want your app to use Gtk+ on Linux and be reasonable elsewhere, consider wxWidgets.