Skeleton, keep it simple. The shape of the ribcage is of more interest (at this point) than drawing each actual rib. It's a huge barrel, just be sure to follow the form of it. Study the skeleton of your animal, measure the skull and compare it it to the lengths of the other bones in the body. This will help you make more accurate drawings. Students of animal drawing tend to draw the gorilla's arms a little short at first.
(The cervical vertebrae are much longer than a human's for more surface area for the muscles of the neck to attach. Scapula come forward more than humans, cross your arms in front of you - that's how gorillas' scapula sit naturally as shown below.
Gorilla surface anatomy is very similar to human anatomy.
The muscles labeled below.
...And all of that stuff underneath fur
To keep you going, I have added some gorilla models sheets from Walt Disney's Tarzan (1999) (Hehe:)