It's hard to say. The modern equivalent of the GAU-90 is the Vulcan 20mm electric cannon, which is a scary powerful weapon that can tear up a car like it was made of tissue paper. The GAU-90 fires bigger bullets but has a slower rate of fire. The wiki mentions that it can fire a variety of rounds like explosive and armor piercing.
I honestly don't know if those rounds would hurt Cage. The Marvel wiki quote says he can take handgun fire from a range of 4 feet, but that would be like a spring shower compared to the GAU-90. However it also says he can take a 150 lb blast of TNT, which would turn the AMP suit into so much twisted metal. I reasoned that, since an AMP suit is able to stand up to a few seconds of GAU-90 fire, Cage should be able to take a lot more.
Yah all good points. Most likely the rounds wouldn't hurt Cage. The problem is that there isn't a lot of specifics given on the suit.
Anyway, here is some additional info on the bullets that i found on the avatar wiki
The GAU-90 eschews modern caseless ammunition, instead relying on tried-and-true cased rounds. While this reduces the effective weight of ammunition the AMP suit can carry, the reliability of cased ammunition is much valued in the corrosive Pandoran atmosphere. The simpler propellant used in cased rounds offers the advantage of greater shelf life and stability compared to caseless rounds.
The hot brass casings ejected from the weapon weigh 2 lbs (0.91 kg) each and often land up to 30 ft (9.14 m) from the weapon, which can prove problematic for nearby unarmored ground troops.
It is used as a "rifle" by the Amplified Mobility Platforms. Range and projectile ballistics are reduced due to Pandora having a denser atmosphere than Earth. However, given the often point-defense encounter profile, this is not seen as a serious disadvantage.