More often than not, the most disturbing horror films are synonymous with their gruesomeness. Movies that focus on gore over narrative tend to have nauseating consequences. Sometimes the story is genuinely frightening, but the grossest horror movies don't need jump scares or hidden monsters. Cannibal Holocaust is hard to watch because of its realistic rotting flesh, and The Human Centipede is notorious for its reputation as one of the most gruesome movies ever made. Neither of these films, however, is known for being unsettling because of its plot as much as its gore.
There are plenty of horror movies that aren't scary, but just because some narratives have been played out doesn't mean all incarnations are doomed to fail. There are far too many patient zero stories out there, but Contracted is just as difficult to finish as any other film about an unknown virus, this time because of the carnage rather than the hopelessness of the premise. The grossest horror movies that aren't actually scary are often still worth watching - but use your discretion, because these are not for the faint-hearted.