Yeah, anyone who goes outside is dead. Just tape all cracks -- door and windows and apply blankets or towels to the bottoms of any really wide openings you can't seal with tape or plastic. If your house is average and you do this, unless the poison is 100% lethal in any dose, you have a chance of survival.
The truth about most poisons is that there's a lethal dose. The dose is the poison. For those that are really lethal in very small doses, there's no need to do anything, b/c any exposure is going to kill you. Find your loved ones and tell them you love them and make your peace with whatever you believe in.
For those that will blow away in 3 hours, even something quite deadly, once it is gone, you aren't gonna die unless you're in it. Then stay indoors, and keep it from coming in. The key is to avoid exposure while it is out there. If you don't know what it is, barriers to entry are your best defense. Some poisons can be amplified by wet towels b/c they react with water to form an acid (for example). If it's something that will not react with the water, the water may help make the towel a better barrier by absorbing some of the gas. Look out for pinpoint pupils, urination, defecation, dizziness, ataxia in others, and death of small animals around you. Smaller things usually will succumb faster.
Panic is your enemy. You can do without water and food for up to three days if you're in good health. Air, about 60 seconds. Once respirations cease, it's six minutes to brain death. Water, you can go without drinking for 3 days, but you can get pretty ill by then. most normal people can go without water for a day without harm -- ask the religious -- many religions do 24 hour fasts. You can go without food for as much as 40 days, more , depending on your health, especially if you're not moving much. So worrying about stocking up? Not that important if the threat is gas outdoors.