Nerve gas would've also killed the defenders, since just ducking under cover wouldn't have protected them.
Actually I think Guy
is talking about the bad guys using the nerve gas instead of sending in the alien death army becuase they want the town empty so they can look for the magufin thingy.
Besides, look at how they've had to build a makeshift community from scrounged resources, what was left over in the ruins of the old civilization. Why would they even have nerve gas?
Though that is a valid point.
Another one is how do you know nerve gas would work on any of the aliens races and if it did if it would work on all of them.