I was in Ruanda in July 2008.
The Gorillas (Group 13) was just behind the border of the national park.
The gorillas were in an open clearing in bright sunshine.
The clearing was small. The distance was about 4 to 5 meter.
Some small gorillas played and have been "rolled" about a half meter in front of my feet.
We photographed in a line. Suddenly, a Gorilla came from behind and has pushed through the line of people.
Some people hat body contact.
I have used to cameras:
D200 with 18-70 3.5/5.6 (or was is already the 16-85?) (about 30%) and
D300 with 70-200/2.8. (about 70%)
During the visit, I have included a TC17 for Portraits.
The 200-400/4 stayed in the hotel, to reduce mass
I bought the 70-200/2.8 for this trip.
However, I have used the lens on all following trips.
It is still one of my favourite lenses.
In the bright sunlight f/2.8 was not needed.
But the gorillas can be in dense vegetation and it can be a dark, cloudy or even a rainy day.
Dynamics - the distance between white and black - is very important in the rain forest.
High ISO reduces the usable dynamic range. f/2.8 might help here, too.
I recommend to shoot in raw (or raw + jpg) for the gorillas to have more freedom in post processing.
I recommend under exposure.
Gorillas are black. But the hairs have reflections in the sun.
Without under exposure you will get "grey" gorillas.