Toxic Gadget Pocket is a gear perk that lets you carry more poison grenades and poison proximity charges. It gives a poison build more ways to apply the poison status in each fight.
You hold more poison throwables. You can gas several groups before you need to restock.
Wear Heisenberg's Jacket in the chest 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.
Snakebite adds damage against poisoned targets, and Relapse heals you when you kill them.