You can download it from gtk.org, for an overview see wikipedia:GTK+. Precompiled libraries for windows can be found at gimp.org/win32, among other places. The preferred way to obtain GTK+ in binary form for Windows NT 6 is to use MSYS2. The last version supporting Windows 2000 was 2.16.6 (2010). We've had trouble tiptoeing around bugs in different versions. See Gtk minefield.