Thats the unique thing about TOI, which is different than other games where the AT gun and crew are an inseparable unit. In TOI the gun is just a piece of equipment, and whatever squad is available can be make the gun operational. Thats actually more realistic, but it can get confusing which squad is able to operate the gun.
Now I thought about having a base made for the AT guns, which the gun stays on, but has a couple of holes for figures to be placed in like a regular squad base, and in effect this is an AT gun squad, which is its own unit. The unit would take casualties like a regular infantry squad, and when the last infantry figure is eliminated, the AT gun base can be either reinforced by another squad transfering its infantry figures to the AT gun base, if in the same hex, just as they could for another squad.
The AT gun on the base would not be considered a vehicle trait but rather the infantry or soldier trait, and would provide a +2 cover as long as the AT gun is not selected for casualties. The AT gun would be worth 2 casualties, and the entire base would have the AT gun, plus 3 holes for infantry figures, thus requiring 5 hits total to destroy to eliminate the AT gun base too.