Sir.
Explanation: This joke humorously plays on the idea that a 600-pound gorilla would be a fearsome and authoritative figure, suggesting that anyone encountering such a creature would simply address it as “Sir” out of respect or fear. The punchline cleverly contrasts the absurdity of a gorilla wielding a shotgun with the polite formality of addressing it as “Sir.” The humor arises from the unexpected and comical juxtaposition of the gorilla’s size and weaponry with a formal greeting.