Don't they have them playing already??
Seriously though, of course not!
Firsty, he's genetically modified. OK, in this instance he's genetically modified for extra intelligence, not extra strength, but once you open that floodgate, there's no going back.
Secondly, Football, as with most other sports, is supposed to be a contest between Humans (or at least within a single species). For example, why couldn't you let a Cheetah enter Horse racing? Aside from the fact that one couldn't carry a jockey, and would probably attack it's competitors, it would be much faster. We've always known that the animal kingdom is more adapted to thier environment than humans, and we as humans have attempted (poorly) to emulate thier abilities. Develop a super-intelligent seal, and there's your next world swimming champion, until someone develops a super-intelligent Penguin!
Thirdly the risk! While the gorilla may state that he'll never intentionally hurt another player, Players get hurt all the time as it is now. All professional players are atheletes, competing with the best in their sport. It's unreasonable to think that if you suddenly widen to a large extent the variation between levels of strength,there wouldn't be consequences. Injuries will happen, more frequently and more seriously. Imagine having a team of Pee-Wee Hermans, playing a team of Pee-Wee Hermans, but one team's got a current NFL linebacker. Are we getting an image now?
Steer the Gorilla towards Professional Wresting. He'll feel much more at home, the risk to humans will be less, and he'll be competing agains his own kind!