Poison gas is a random event from Thieving chests. Occasionally, while looting a chest, gas will come out and damage nearby players. Despite the fact it has a green hit splat, the event does not poison you.
Poison gas is also found by killing the Undead Ones on Karamja. The corpse will release poison gas that will damage all players near the object for 2-3 damage. Simply walk away to avoid further damage and wait until the gas stops. Anti-poison potions are not required.