Gorilla's have a really big gut. Meaning, they have a really long digestive system that they absolutely need to break down extremely fibrous vegetable matter. All herbivores have very long and sometimes complex digestive systems. They need that to convert all that vegetable matter to usable nutrition.
All carnivores, like big cats, wolves, etc. have much shorter digestive systems. They take in muscle meats, organs and bone marrow, etc. that herbivores very considerately have already converted into easily digestible proteins. Human beings have digestive systems much more similar to carnivores than herbivores, although ours is a bit longer. We are more properly omnivores. We can't survive totally on a vegetable diet without supplementation, but we can obviously use vegetables.