gorilla mask is a reward from The Pit. It is obtained after successfully completing one activity in The Pit, and talking to the Talent Scout at any agility course except the Burthorpe Agility Course. If you do not have a free inventory slot after you do a minigame from the pit, then you do not get the gorilla mask.
You can enter the game by completing laps on an agility course until the talent scout calls you. Speaking to the Talent Scout with an
Agility cape, Max cape or Completionist cape equipped will allow instant access to the game, and a mask can still be obtained for this.
7 February 2012, the gorilla mask can be added to the Costume Box in the player owned house.
If you visit the Talent Scout on Ape Atoll, you can see him wearing this mask to "blend in".
If you get spotted while wearing the mask in Ape Atoll, in the Jail your character will say "I was sure wearing that gorilla mask would work."
When the Talent Scout hands you this mask, he will say "there is no need to be upset", making reference to the internet meme on which a gorilla appears on a picture saying this.