They would make great infantry, and would pose a serious threat on the battlefield.
Gorillas are stronger than a human being, and capable of carrying far more equipment/armor.
Each of them could easily carry an LMG + ammo, while a regular human soldier would only have an assault rifle and far less ammunition. Having to reload less often, and being able to sustain fire for longer would definitely give them a tactical advantage.
Last but not least, if they engage a human in hand to hand combat the human will, unless very lucky, lose. In an arms ripped off kind of way.
Lower Profile & Increased Stability
Gorillas are shorter than us, and that actually gives them a fair advantage in an urban setting. A low profile is a thing to be desired when you've got snipers and the such lurking in the shadows.
This also ties in with the fact that Gorillas are designed to operate in mountainous regions. They will excel at climbing over rubble, making their way through ruined buildings, etc.
These Gorillas will move faster than human infantry, carry more firepower, more ammo, and more armor. I pity the poor bastards going up against them.
Edit. Or Why Gorillas as a Replacement for Humans Is Silly
I'd like to address some of the points which other people are raising, and which, while relevant, I do not believe are deal breakers:
A human may very well have more endurance than a gorilla - I actually have no idea. However, why would gorilla infantry be used in exactly the same way as human infantry?
Everyone seems to be thinking of gorillas as replacing humans, which to me, is silly: there's lots of military gear that would be too expensive to modify for their use. Instead, a trained gorilla soldier would make a perfectly complement to a human unit. Gorilla units would be used as heavy, shock infantry, right alongside conventional human units.
They could be used as support troops, capable of emplacing heavy machine-guns, or carrying mortars and heavy loads of ammo close to enemy lines, or even behind enemy lines.
How much endurance they have is irrelevant! They can last long enough to do terrible damage to the enemy, at which point you pull them back for R&R, and allow the more rested human troops to push forward.
In this day and age we are capable of condensing a thousand or more calories in something the size of a granola bar. Are you trying to tell me that feeding the gorillas, while expensive, would be a deal breaker? Really? No.
Aiming & Weapon Handling
Frankly, I don't see what the issue is. Humans run from cover to cover and shoot from behind said cover. Gorillas would be perfectly capable of doing the same. As far as having to unsling their weapons while they run, I don't see a problem: gear already exists which allows troops to do so in seconds, not to mention that special weapons (wrist mounted solutions, etc.) could be developed for them so they are not defenceless when their weapons are slung.
Gorillas don't have to behave - or be deployed - in the same way that regular human soldiers are. They may not be able to fill the exact same roles, or perform in the exact same way. Like any weapon, they have their strengths and weaknesses, and that's fine. The trick is to use them in such a way that you gain an advantage over the enemy, and I believe that their value as shock troops / heavy infantry is simply over 9000.