Grey back gorillas are not vegans or even vegetarian. They eat insects, caterpillars, and termites as well as plants and fruits. Also, some of the leaves they eat have a surpsingly hefty amount of protein.
Most animals who graze get a great amount of protein through insects, and their bodies utilize the protein in leaves and such in a different way than humans do. I don't think we can really compare our diets.
Further, gorillas nurse a good amount of time. As I said, I am not talking about adults, I am talking about human children who grow at such a rapid rate, and need to grow large brains-- and therefore do have different nutritional needs as compared to a full grown adult.
Frankly, I think the ideal human diet matches the primate one. Heavy on dairy in childhood, mostly vegan with a small amount of animal intake in adulthood. But neither vegans nor omnis want to accept that, because 1) it would mean vegans would have to accept eating a small amount of animal products and 2) omnis would have to abandon meat as a frequent meal staple and eat it much more rarely (as in once or twice a month), and, they would have to give up dairy foods as adults.
It is common for the young of a species to eat differently from the adults of that species. I believe there is a type of ant that is carniverous as young but vegan as adults.