They live in a dense planetary ring
In general, a body plan's relation to how much it can scale up in lower gravity is directly proportional to height. So if you assume gorilla like adaptations, but 300 times as tall, then you need 300 times less gravity which is about 0.3% of Earth's gravity. Unfortunately, this indicates something in the gravity range of an asteroid roughly 100km across rather than anything massive enough to be even remotely planet like. Such a celestial body can not hold an atmosphere meaning your giant gorillas must be anaerobic (so you should probably ditch the heart shaped noses all together). Being anaerobic also solves some of the major issues with the oxygen density required for such a large respiratory system.
However, an asterial body this size does not have enough landmass to feed a population of them. A single family of gorillas on Earth needs about 4-30 km² of wilderness or 0.04-0.3 km² of dedicated farmland to survive. If we assume similar scale megaflora, then that means a single family of your aliens needs about 360,000-2,700,000km² of wilderness or 3600-27,000km² of dedicated farmland, but a planet with low enough of gravity for these guys to live would need to have a surface area of about 32,000km² or less. This means that even with advanced agriculture, a planetoid small enough for them to live on would could not support more than about 40-50 of them, and they never would have evolved to be able to get that far.
So, your best bet for these guys is not to stick them on a single planet, but for them to live in a part of space where there are billions of these tiny planetoids that they can jump between. This way, they never experience crippling gravity, but they have access to tons of surface area that they can cultivate thier space veggies on.
It is possible that these creatures started off normal gorilla size when thier moon fell past the Roche limit of the gas giant they were orbiting, but the breakup was slow enough that they, and the plants on thier world had time to evolve. They've all gone super-sized route because this helps the giant animals leap between large rocks that are many km apart, and the large plants that feed them have been selectively bred by the gorillas because the gorillas only transplant flora that are large enough to make for good eating.