Gorillas are big and strong. Like most of the big and strong land animals, they eat leaves. Animals that mainly eat leaves are called folivores. (Foliage means leaves.) In this article (http://www.jstor.org/pss/4219431), some biologists explain why you would expect a gorilla to be a leaf-eater:
Large mammalian herbivores, according to accepted ecological theory, may be expected to feed on abundant, low quality food as a result of the relationship between their body size, metabolic requirements, and gut capacity. … Abundant low quality food means nonreproductive plant parts, i.e., stems and leaves, because fruit and flowers are too ephemeral to provide a regular source of food in bulk for large animals. So, large herbivores ought to be folivorous in a broad sense, and usually are.
Really big animals need to eat a lot of food. Flowers and fruit may be tasty but are in short supply. For this reason, the really big plant-eaters tend to eat mostly leaves.