My vote is for MEN BEHIND THE SUN. It might not be the "goriest" movie although it is quite gory. It has to be the most disturbing movie I've seen (especially cause its a true story)

Here are a couple of scenes. Don't watch if your even the least bit squeamish.

http://www.youtube.com/watch?v=BGPMEtfEKMY

http://www.youtube.com/watch?v=RZU0tvn1ds4