I strongly believe a Bear would easily win. It has stronger Attack and Defence.
It has massive claws and will shred open a gorilla's unprotected skin with it's claws and teeth.
A gorilla has very little in terms of defence and that will be its downfall, it also only really has its teeth to attack with, and a Bear has thick skin to protect against that.
I don't see how a Gorilla could deal a fatal blow to a bear before the bear would rip deep wounds into a Gorilla with it's claws and teeth causing it to bleed out.
Yea there is no perfect solution, but we shouldn't be looking for perfection. Every step in the right direction counts