It turns out that gorillas' diets really do consist mainly of plants! Although there are slight diet variations depending on the type of gorilla. Western lowland gorillas' diet consists of 67% fruit, 17% leaves, seeds, and stems, and 3% termites and caterpillars. The diet of Mountain gorillas, on the other hand, consists of 86% leaves, shoots, and stems, 7% roots, 3% flowers, 2% fruit, and 2% ants and snails. The lack of fruit that the Mountain gorilla consumes compared to the Western lowland gorilla is due to the scarce amount of fruits that grow at the high altitudes of their mountain habitats.
Gorillas are, in fact, able to survive on only fruits, leaves, trees, and other plants. However, in order for them to obtain all of the energy and nutrients that they need, gorillas must consume a large amount of this vegetation, which, luckily, is available to them year round in their natural habitats. Adult male gorillas consume more than 40 pounds of vegetation every day! Now that's a lot of salad.
Here is a cool video where you can see gorillas eating up close.
Gorillas use their strength to break apart plants. They are also very selective foragers, in that they like to only eat certain parts of plants (which is what I had noticed at the zoo!). This helps so that they don't overexploit an area for food. They crop the vegetation in a manner that allows for quick replenishment to occur. This system is very important because this is their only diet, so they need a constant, abundant source if they are to survive.