Actually, we've had a tester in the Octagon already with Brock. Apparently gorillas are 10x as strong as an average man which is insane considering they only eat veggies and fruits.
A healthy man can bench his own weight, the average being around 185 lbs. A gorilla, highland silverback, the biggest of the gorillas, is said to be between 12 and 20 times stronger than a man. Just multiply 185 by those figures and it will give a ball park figure of what a gorilla can do if taught to bench press.
On the animal channel, it stated that a large male silverback gorilla has the strength to press over 4600 pounds.
So, the gorilla probably will just rip the arm off. I've visualized this scenario time and time again; leapard, bear, gorilla, tiger, lion, etc. I'd probably go for the RNC hold. Can't out muscle an animal unless you poke it's eye out, but considering you want to fight fair...lolz...RNC is the way to go.
Oh and since this is a Ronda thread. I'll say Ronda "might" be able to pull off an armbar on a a baby chimp or orangutan.