No "of" is not required. It's optional, but I would not use it.
I'm not sure the logic is entirely right with the "unlike." Are gibbons unlike the others because both sexes are the same size? It seems a bit off to say that one way they are "unlike" is that they are "smaller."
I prefer: "Gibbons are much smaller than gorillas, orangutans, and chimpanzees; and both sexes are the same size.