Probably the grizzly. Bigger, heavier and a natural killer. I remember reading about when bears and lions used to be made to fight for entertainment, and a grizzly bear could kill an African lion with a single swipe to the skull, so I imagine it could also inflict serious damage to a gorilla if it can land a blow. The gorilla is smarter, more agile and incredibly strong, but I'm not sure it could overcome the size disadvantage, plus it doesn't have the nature of a predator.
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