Humans produce a protein called myostatin that inhibits muscle growth; it makes it difficult to grow big muscles. Having too much muscle slows you down and tires you (and your heart) out. That protein limits muscle growth so that humans don’t need to consume ridiculous amounts of anything and can survive when resources are low.
Gorillas don’t have that protein.

It’s beneficial to limit muscle growth because of the high amount of calories needed to sustain them. If you’re not using them, you lose them.
We’re made for running, not lifting ourselves up on trees. For that, we needed to be leaner and lighter. Being heavy and muscular only slows you down when you’re hunting gazelles.
We evolved for endurance running, rather than for sprinting speed. Even the fastest human or pre-human primate ancestor would be unable to catch large prey over short distances. What humans evolved to do effectively is manage heat through sweating and lack of fur, which allows us to chase prey until the animal is exhausted or collapses from heat.

You probably noticed that many animals have considerably more muscle mass than humans. This is not only limited to gorillas, but you can also see it in bears, cows, bulls, horses, big cats, boars, just to list some you may know. This is mainly due to genetics.

Every animal is built for a different purpose. A human is built for endurance and fine movements, a gorilla is built for using its power. It must be strong enough to climb trees easily, despite its weight. An average sized male gorilla who has the same height as a man, can easily outweigh the man three times. Remember, both being average and not extraordinary.
A 500 pound gorilla has enough strength to climb a tree with the ease of a ballet dancer. Just as you have enough strength to carry bags home from the grocery store. Or enough stamina to walk 50 miles without training.
The gorilla doesn’t need to work out to get strong. It must be strong, therefore it is strong. It does calisthenics on a daily basis. It has more fast twitch muscle fibers and they are stronger than yours. The gorillas bone density is much higher, the bones are thicker, the heart and lungs bigger.
You (a human) don’t need to be physically strong to survive. You need to be smart. Excess muscle is a waste of resources, especially if you don’t need it. If you stop working out, you will lose most of your hard earned muscle. The gorilla will not.
To put things better in perspective compare the frames of a man and a gorilla.

By the way, powerlifters also spend a lot of time sleeping and eating. They don’t work out 24/7.
Dexterity, strength, endurance. Select two, cannot get third.
In evolution, all attributes are trade-offs. Evolutionary pressure has guided gorillas towards brute strength on cost of dexterity and endurance. Gorillas cannot swim, humans can.
Gorillas and humans simply occupy different ecological niches. Humans are persistence predators, and as such, dexterity and endurance are of much more avail than brute strength. Gorillas live in dense woods, where strength is an asset.
Humans also have the keenest eyesight of all mammals and second only to diurnal predatory birds in the whole animal kingdom.