Do You Think a Gorilla Would be a Good Killer?
Naturally just shoving a Gorilla by itself isn't a good killer, obviously there would have to be some primal based power and some other elements to make them horror. But to make the argument FOR a basic Gorilla we can take the fundamentals of horror antagonists and make them apply here.
Horror relies on the concept of 'the other' or some creature that is unnatural to a circumstance or within an entire setting and situation that is unnatural to the protagonist (Which should be a natural extension of the audience.) That's why the less we know about it the better. For example, if you put an Orc in a fantasy setting, that's just a normal every day occurrence. You put an Orc in a modern day setting that doesn't typically have orcs, and have it hunt in the night you have horror. I'd say that the latter example is very similar to your slasher horror.
So by that context alone a Gorilla outside of its element in a situation that is intrinsically not natural or not safe for humans can easily be horror. Narratively speaking this would be a classic story of Man V. Nature, this isn't that uncommon for horror either. If you look at Western's a lot of horror found there was in the wildlife like bears, wolves and mountain lions.
A lot of the killers we have already have elements of not knowing it's motive like Dredge and Demogorgan. Essentially primal forces of nature that just kill for the sake of killing.
I also did a quick google search and discovered that a Gorilla's strength can be as much as 4 times that of a human which got me thinking. A 4v1 is a perfect match against a gorilla which is the asymmetrical formula that DBD uses.
I don't necessarily know what the power of a Gorilla could be, but I do know that I would be terrified if I were being pursued by a Gorilla especially in the indoor maps.