Start with a silverback gorilla, make it roughly three times bigger, assume it's bone and muscle structure is capable of compensating for the increased size: how fast can a thing like that reasonably move, when it's not sprinting but it definitely has somewhere to be? Would it be faster or slower than a human moving on foot at the same level of urgency, and is that linear or does it depend on the speed?
3 times bigger than a gorilla is quite big, too big to fast but big enough that it needs less steps to reach the same speed.
Looking back in history some members of the ornithopoda were decently big, some way bigger than 3 times a gorilla and they walked on their front fingers which made them really slow, but they could gallop at high speeds when standing on only the back feet.
You need to understand that knuckle walking is just an evolutionary step between quadrupled animals and bipedal ism.
In the case of ornithopoda those dinosaurs where slowly transitioning from bipeds to quadrupeds, the opposite of gorillas.
So I guess a giant gorilla would adapt a new style of movement and would be really slow both when walking upright or on knuckles due to its sheer weight.