Dress for the forest, not for the photograph
Closed walking shoes with good grip, long trousers, light layers and a waterproof outer shell are more important than heavy safari clothing. Forest trails can be muddy and vegetation can be wet even when it is not raining.
Carry only what you need
A small daypack, drinking water, camera, spare battery, insect repellent and sun protection are enough for most trekkers. Porters can be extremely helpful, especially if you want to protect your energy on steep terrain.
Keep flexibility in mind
The gorillas choose where to move, so trekking time varies. Pack to stay comfortable for a longer walk even if your group eventually finds them quickly.