Gas is a Composition Type that makes a creature immune to all physical attacks and gives them a +20% miss rate for magic. Creatures with poison attacks also give off a poison vapor in squares immediately adjacent to it. Gas creatures have a +15% chance of turning invisible.
On the other side of the coin, gas creatures have no physical attacks and can only cast magic if the creature has magical abilities. Transmog a Cyclops into a gas creature, for example, will render them completely harmless.