You can teleport to them under "Monsters" at the teleport mage
Inventory & Gear
You want 1 super set and a few prayer potions and rest food
Bringing range is recommended, since demonic gorilla's prayer protection do work.
As for gear Slay helm is recommended on task. Else you can else Serp, neit or barrows helm.
I prefer to use Hasta, but whip is perfectly fine too.
Blowpipe -> crossbow -> MSB / dart / knife for range secondary
Karil -> Black d'hide body -> or tank top to switch when meeling
Tassy -> Barrows -> dragon -> rune legs
Ava -> firecape to keep bolts/arrows
Strategy
Gorillas always attack with the same style when they pray, so you can counter this by switching your pray to take less damage.
When Gorllias pray Mage they attack with Mage
When Gorllias pray Ranged they attack with Melee
When Gorllias pray Melee they attack with Ranged
Their prayers work, so remember to switch between range & melee, else you won't do damage.
Drops
Zenyte Shards (for torture/Aungish etc.)
Balista Parts