The outside.
Explanation: This joke uses a play on words to create humor. The punchline, “the outside,” suggests that the gorilla’s hairiness is a characteristic of its exterior, which is true since gorillas have thick fur on their bodies. The joke is funny because it subverts expectations; instead of a more complex answer, it provides a straightforward and somewhat literal response. The humor lies in its simplicity and the cleverness of the wording.