When selecting lenses many people elect for long telephoto zooms when travelling on safari. While these are often the best tool for the job when you are in somewhere like the Serengeti, they can be a bit too much when visiting the mountain gorillas. The reason for this is that, ideally, you should be around 7 metres from the gorillas, which is significantly closer than you are likely to be to a leopard in a tree. If somebody else was paying for it (and carrying it!) I would love to go with 2 x Canon 1dx bodies fitted with Canon EOS battery grips, a medium telephoto such as a Canon EF 24-70mm f/2.8L II USM coupled with a Canon EF 70-300mm f/4-5.6L IS USM. But as this blog and my trips are based in reality, I went with none of those things whatsoever. It is all well and good saying that this is best or that is the best, but I shot exclusively on a Canon 50D with a Sigma 18-250mm f/3.5-6.3, which is significantly cheaper, and lighter.
Essentially, what I am getting at here is that you should take the best equipment you can afford but try not to get to too hung up on equipment and spend some time learning how to use your system and this will get you a lot further than just spending money on expensive kit.
Other than that, plenty of memory card space, additional batteries. Check that memory cards are empty and batteries are full before you set off, and give your lens a nice clean before you leave as well.
Tripods seemed very popular amongst other people in my group when I visited the gorillas, maybe I am missing something, but I really did not see the point. Why would you need a tripod when you are shooting something so close.... that moves. It is rare that a gorilla will sit still enough to allow you the benefit of shooting with a slow shutter speed so what is the point of the tripod? A tripod worth having is absurdly heavy and gets in everyone else’s way, if someone can tell me the benefit of this that would be great! I am a big believer that hand held is the way to go, increase the ISO if you need to shoot at higher shutter speeds but I would personally avoid restricting myself with a tripod.