I personally think the exact opposite. GTK needs a lot of custom styling to get the "native" style to work, and even then it's kind of lacking.

GTK feels right at home in the GNOME desktop environment (which I use, and like!) but for cross platform I don't really think it feels native at all. As a user, I've had much better native experiences with QT or WxWidgets to be honest (on Windows, at least).