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Poisons the target. Pokemon protected by Magic Coat or the ability Magic Bounce are unaffected and instead use this move themselves.
Poison Gas is not a good move mostly because of the existence of Toxic, which is not only more accurate and more available, as almost all Pokemon are capable of learning it through a TM, but the toxic poison status that Toxic inflicts is undoubtedly more desirable than the regular poison inflicted by Poison Gas. Of course, Poison Gas does hit all opponents in a double battle and all adjacent foes in a triple battle, but in those circumstances there are much more potent status conditions to use regardless. Also, Toxic Spikes does a better job of reliably poisoning all the foes on the opponent's team in one stroke, barring those that are already out.
Toxic is a better alternative, as it inflicts Toxic poison instead of regular poison.
PoisonPowder is an equivalent, but with slightly lower accuracy and PP.