So the Gatling gun fired the same round as a Springfield rifled musket, but at a constant rate equal to that of ~50 men (150/min). A Gatling unit in AACW has (I think) 12 pieces, making it the minute equal of 600 men. While the Union did not use a lot historically, they were available, and other rapid-fire guns were also available and used.
While the Williams gun did fire a larger round than than the Gatling, it was also much smaller than other period artillery. This limited it to firing solid shot and canister, which are less effective at range against infantrythan shell/case shot. Likewise, its effective range was much less than that of the Napoleon. It's rate of fire, while several times greater than other artillery, was also several times smaller than the Gatling etc guns. The greater penetration of a 1.57" ball vs a .58" ball against infantry isnot terribly important, as both could disable a man, but the line style of combat would limit tge damage caused as the ball continued beyond the penetrated soldier.
So, if this gun were represented in the game, it should probably have the same mobility as horse artillery, a range of 5, a RoF of 3, and damage similar to infantry (2/20), maybe 1/15 at range.