Indeed. He should use wxwidgets (with c++ for instance) or something else better (than gtk+). As an extra he'd get the programs look less alien between platforms. It would mean more satisfied users via the pleasant look and decrease of usability issues due the inconsistency of the guis in use.
(Especially the gtk-wimp is horrible on Windows, it's nowhere near the present Windows widget and look set.. It simply looks and feels/acts wrong, especially when sided with the newer 2003 and 2007 style blue widgets. Gtk+ applications are plain sad on Windows.)