The Win32 port of GTK+ is maintained concurrently with the Xwindows version in Git. As such it is a supported architecture.
The Win32 port has been predominately done by Tor Lillqvist. Tor maintains some information on the Windows download page (32bit and 64bit).
There is a pre-compiled, easy-to-install version of GTK+ for windows on the Dropline GTK+ site.