Final answer:
When chimps and gorillas are on the ground, they principally move by knuckle-walking. They have adaptations for arboreal movement, but on the ground, they walk on their knuckles and feet in a quadrupedal stance.
Step-by-step explanation:
Chimps and gorillas are indeed suspensory when in arboreal contexts, using their limbs to swing from branch to branch. However, when they are in a terrestrial context, they are principally locomoted by knuckle-walking. This means that when they are on the ground, they support their bodies with the knuckles of their hands as well as their feet, giving them a stable quadrupedal stance which is effective for navigating the terrain. This method of locomotion is distinct from brachiation, which is swinging through trees using the arms.
Both chimpanzees and gorillas have adaptations for climbing trees, as do all primates due to their arboreal ancestry. This includes limb adaptations such as a rotating shoulder joint and the separation of thumbs and big toes to effectively grasp branches. However, when it comes to terrestrial locomotion, they shift to using knuckle-walking. These great apes are part of the Hominidae family, which also includes orangutans and humans, although only humans use bipedal locomotion as their primary form of movement on the ground.