Change your preferences here.
Poisons the target.
Poison Gas is not a good move mostly because of the existence of Toxic, which is not only more accurate and more available, but the badly poison status that Toxic inflicts is undoubtedly more desirable than the regular poison inflicted by Poison Gas, despite turning into normal poison after a switch out.
Toxic is a better alternative, as it badly poisons the target instead of regular poison.
PoisonPowder is an equivalent, but with slightly lower accuracy and PP.