Please note: by reading the advice below, you may get the impression that primate tracking is a dangerous activity. This is not the case. Tracking gorillas and chimpanzees can be considered very safe as long as you treat them with respect and use common sense. It is extremely rare that dangerous incidents occur, and your professional guide is there to ensure your safety. The advice below is solely intended to further lessen the already minimal risk.
Mountain gorillas are, in essence, gentle giants. They show very little aggression toward each other and their overall vibe is extremely peaceful. Chimpanzees are, by nature, a lot more aggressive but the groups you can visit are habituated. Aggression towards humans in that context is extremely rare.