Strong Constitution is a gear perk that protects your eyes from poison. Poison gas blurs your vision, and this stops that effect so you can still aim through a cloud.
Poison no longer blinds you. You keep a clear view while the gas still hurts, so you can shoot your way out.
Wear the Hazmat Mask, Heisenberg's Porkpie Hat or Protestor Hat in the head slot. Gear perks are fixed to the piece of clothing that carries them, so you get this one by wearing that piece, not by unlocking it in a menu.
Poison Protection reduces the damage, and Poison Rush gets you out of the cloud faster.