I can't seem to find a Gore-tex hoodie either mate.
The closest and lightest thing I can find are Gore-tex zippable hooded jackets, like this:
http://mountainwarehouse.com/shop/pr...s_id/5885.html
Tried looking at a few outdoor clothing/military clothing shops, but alas no proper 'unzipped' hoodie tops.
Even tried searching for a Gore-tex kagool (which I suppose are similar) but not joy.
Not sure why they're so elusive, but I agree they'd be a great piece of kit.
Maybe they're all zipped as they're designed for outdoor use and designed for access to inner-layers/pockets etc (just guessing here).
They're a bit pricey too lol.