The Win32 port of GTK+ and gnome was a complete afterthought of these libraries. In this case, the core library interface didn't change at all and was ported to windows quickly.
Gtk/win32 has NOT native look and feel. Mistakes like the one of gtk are what we are hoping to avoid. Suppose you want the feature "applications running in the bottom bar" (say systray). You implement it just for win32, and then... you have no way to implement the functionality in Xaw, at least not without adding a panel to the Xaw environment, thus forcing a look and feel that is not native. I guess this is what other people mean when they speak about native look and feel, and the necessity of having a good number of backends in mind when designing the interface. Vincenzo