[octothorp]

If we're talking about a fight to the death, I think the thing to look at is the scenarios in which one animal can realistically kill the other.
Polar bear: polar bears are very good at fatal attacks: they have strong, compact arms that are designed to crush bones and skulls with a single swat, and a claws that result in deep penetration wounds. Against a gorilla, it's unlikely that such a swat would knock out a gorilla: they have massive and strong skulls. But they're definitely vulnerable to having limb bones broken by such attacks, and of course very vulnerable to flesh wounds.
The second killing technique that the polar bear has is its jaws. Again, it's questionable whether the polar bear could actually crush a gorilla skull, but it can probably cause enough bleeding to kill it pretty quickly. As well, a polar bite on a neck will be almost immediately lethal. A polar bear bite on any limb is going to disable that limb very quickly.
Gorilla: Gorillas fight for troop dominance, but much of the fighting is of a non-lethal variety. Even against other species, gorillas fight only for protection; leopards have been known to hunt gorillas, and gorillas survive these attacks usually through troop tactics. Defending yourself against a leopard is completely different from actually killing another animal, especially one as powerful as a polar bear.
The gorilla has insanely powerful jaws, but they're designed for crushing vegetation, not for tearing through flesh. Despite sharp-looking front teeth, the gorilla's teeth are designed for crushing; the sharp teeth are there for causing pain in non-lethal fights. While a gorilla bite would hurt and remove a chunk of flesh, it would take several bites for a gorilla to get close to anything lethal. If it got its jaws around a bone it could probably crush it, but the thick mass of fat and muscle around a polar-bear's limbs would make it difficult for the gorilla to get such a bite.
The gorilla would be able to beat smaller animals just on the strength of swats and grappling. But against a polar bear, neither are going to have much effect. It won't be able to grab a limb and snap it in two, or twist the bear's neck.
Pretty much the only scenario I can imagine where a gorilla would actually be able to kill a polar bear is if it was able to get on the bear's back and repeatedly bite the neck until it's able to get its jaws around the spine. By comparison, there's probably about a dozen scenarios where the polar bear could win, through using its jaws to quickly disable the gorilla's limbs, getting its jaws around the neck, or inflicting serious flesh wounds with its claws.
Gorillas aren't designed to be lethal killing machines. Polar bears are.