Well the idea is that the GI / GGI will be able to move around a little bit and duck under their sandbags / walls to take cover from enemy fire, so it really depends on the skill of the person inside the sandbags / walls. Although spraying fire over the top of them will keep the person inside from being able to stand up and return fire without taking damage. The best way to take them out if they are ducking is probably to run right up to the sandbags / walls and shoot over them down at the person inside. This all needs testing and such.
Also we might make it so that bullets can pass through small gaps between sandbags, so a deployed GI is not totally immune to bullets while crouching, but the inaccuracy of enemy weapons at range means only a couple of shots are going to get through. Again, this is not set in stone.
They take a shitload of damage is what happens. Try not to get HS by a tank, or anything else for that matter