AGGEMAM said:
Aquired immunities should not work retro-actively neither should they suspend effects.
In gaseous form you are just as much a whole person as you normally are.
It wouldn't be retro-actively doing anything. If there are poison effects that haven't taken place yet, those poison effects won't start until after the person returns to normal. A retro-active immunity would mean that the poison would be neutralized and the previous effects would be undone. No one has advocated that position (as yet).
As for the "whole person" statement, true you are still the same race as before and you are still whole. However, your body has been transformed into a magical gas. You no longer have blood and blood vessels. You don't have a heart or a stomach. You're just mist. How can a poison continue to spread and damage organs and tissue? How can you breath? If you poke your head through a keyhole to see on the far side of a door, does the narrow opening start to choke you? No. That's because there is no blood or air flowing anywhere. You're just a gas. So unless it was a magical poison, I really don't see how it could continue to hurt you.
Suppose that you are wearing armor and turn gaseous form. Can the touch of a rust monster damage your armor? I would say no, because it isn't metal now. Would poison continue to hurt you, no, because you have no metabolism now. Also, recall that you aren't only immune to poison, you are also immune to critical hits. Again that is becuase you have no critical spots anymore. You are just a big pile of gas. You have no soft spots with critical organs underneath. You can't cast spells with verbal or somantic components. Why, because you can't speak if you have no lungs, voice box, or mouth with which to vocalize. You cant gesture with your hands if you have no hands.
Now granted, there are exceptions to this. I assume that you can still make use of your eyes since that would be a big enough hinderance that they would have to have mentioned it. Of your 5 normal senses, I think the sense of taste is the only one I would rule as unusable. Maybe smell too. Touch sense would probably be very limited. You could probably only feel if something is very soft or not, and whether it is smooth or not. Since you're not immue to fire & cold, I assume you could feel those as well.