If you want them to work pretty much the same on win32 and unix, I'd suggest cygwin if it's possible to use it. Then your stuff will litteraly run the same. Otherwise small changes will be required on various things.

Other than cyg, I'd suggest using the Tk stuff. I don't like it as well as Gtk personally, but it seems to have better documentation and is more commonly used. Both should run under win32 if I'm not mistaken.