There we go, a complete suite guys! BUT this of course really is PyGI oriented. a.k.a all runtime has been linked to msvcr90.dll using my mingw toolchain (special). Basically it will potentially crashed when mixed with generic dll (linked to msvcrt.dll) so you've been warned!, however it you use complete GTK c runtime package from OpenSuSe or Fedora it should work fine.
As you know GTK-3 is still not production ready (I even found 3.6 to be worse than 3.4 in some case). This however should not prevent you to give it a try :)
GI for windows itself is credit to Dieter Verfaillie for his yet uncommitted patch (more than a year ago) which still relevant today, although GIR generation remain unreliable and messy process under Windows. Also thanks to Helge too for pointing many issues.
http://code.google.com/p/osspack32/downloads/list (look for pygi-aio-3.4.2)
Overtime you will (i'm sure you will) encounter binding errors and stuff. You could fix it without having recompile everything. Just edit (say use gedit) the gir file (located in share\gir-1.0) save the edited gir alongside g-ir-compiler and invoke: "g-ir-compiler -o [binding-name].typelib [binding-name].gir" then copy the typelib into site-packages\gtk\lib\girepository-1.0 and overwrite. As a hint you could grab linux gir version and compare the error part.
I also have static build vala 0.18 to join the party though I don't remember anymore why I did that build:
Oh yeah I'm getting nearer to release the next Tuma MinGW, it should be the biggest toolchain available too.