Though not the largest, grizzly bears are the most ferocious species of bear and can still grow past 1,000 lbs. This is roughly twice the size of a large gorilla!

Plus, Bears have super thick fur which acts as protection and a massive bone structure. The gorilla has no chance...it has no offense which can hurt the bear....you think it can knock the bear out by swinging its fists!?

The bear has claws and jaws made for slashing, tearing, and ripping. It would have the gorilla killed within 5 minutes.

Grizzly bears will more often then not also beat lions and tigers in a fight.

Gorillas are powerful, they would beat the f out of any human, but to question them against a grizzly bear the bear wins 99 out of a 100.

edit:
Also, they used to hold pit fights and have bear vs. bull, bear vs. lion/tiger etc. and the bears would almost always win by crushing the skull of a bull/lion/tiger. SO I think it would have no trouble smashing the skull of a gorilla.