Say what you want this is is an interesting thought, although the polar bear would most likely win because of its sheer mass. The gorilla has a lot going for it. A silverback can get to almost half that weight and has a greater bite force, intelligence, agility, and reach. They can also throw punches of up to 2300 lbs of force, which could easily break a bone or skull.
Gorillas fight a fair amount so to say they have no training isn’t correct.