There are two ways to use gtk+ on win32:
Cut to the chase, show me the binaries! Check out Mario Fischer's Gtk2-Perl site. It provides installation instructions and a PPM repository. Paul Miller provides another PPM repository which seems to be more up-to-date currently. The old "official" repository is also still available if you're looking for older binaries.
There's also a project that provides one installer for everything you'll need: Camelbox.
Compiling from source on windows is no fun, but it is doable. You need Microsoft's compiler, cl.exe, and nmake.exe, since those are the tools used to build ActiveState's perl.exe. Compilers known to work: MSVC6, MSVC7, and the free non-optimizing compiler from microsoft.com (link?).
How to build gtk2-perl on win32, by Tyler Hepworth