Who Knocked Out Gorilla?

Gorilla is the bouncer at Baboon's café. He does his job so well that Baboon is unhappy with him - as are the customers he won't let into the place: Lion, Bird of Paradise. Who is the mysterious attacker who's left him knocked out? The case puts Sherlock on edge as he realizes he's never allowed into the club either. The bouncer systematically refuses to let him in.