A creature in gaseous form has a physical body of sorts, but that body is mostly immaterial. The game doesn't describe gaseous form in the same detail as incorporeality, but the two prove similar in many ways. Here are the basics of gaseous form:
Gaseous creatures have some mass and physical presence, but they are literally as light as air; as a rule of thumb, you can assume a creature's gaseous form weighs about a tenth as much as it did in solid form. Gaseous creatures have no material armor, and a solid creature that becomes gaseous loses all armor bonuses from armor worn, shields carried, and any natural armor bonuses. Dexterity, deflection bonuses, and armor bonuses from force effects still apply to the gaseous creature's Armor Class.
Gaseous Defenses: A creature in gaseous form has damage reduction 10/magic. This reflects the mutable and elastic nature of the creature's gaseous body.
The rules don't specifically mention it (except in the description for the gaseous form spell), but a gaseous creature isn't subject to critical hits.
A gaseous creature does not need to breathe and it is immune to attacks involving breathing (troglodyte stench, poison gas, and the like).
Spells, spell-like abilities, and supernatural abilities affect gaseous creatures normally.
The rules say that winds or other forms of moving air affect gaseous creatures to the extent that the wind pushes them in the direction the wind is moving. However, even the strongest wind can't disperse or damage a creature in gaseous form. Presumably, this means that any wind effect will move a creature in gaseous form. Assume the creature moves 10 feet for each mile per hour of wind speed; apply this movement at the end of the gaseous creature's turn. This movement doesn't count against the gaseous creature's movement for the turn.
A solid creature can't bull rush or overrun a creature in gaseous form (but it might be able to move into its space, see Abilities). A solid creature cannot trip a creature in gaseous form, and a gaseous creature cannot fall.
Limitations: The rules don't say exactly what a gaseous creature can do, but they're pretty clear about what it cannot do. It cannot manipulate objects or activate items, even those carried along with its gaseous form (see Abilities).
A gaseous creature cannot make any attacks (though a gaseous creature may have some special attack form), or cast spells with verbal, somatic, material, or focus components. If a solid creature has a touch spell ready to use, that spell is discharged harmlessly when the creature assumes gaseous form. A creature in gaseous form cannot speak, enter water, or run. When a creature assumes gaseous form, it loses supernatural abilities -- except for the supernatural ability to assume gaseous form and the ability to return to solid form.
Abilities: So what can a gaseous creature do? It can move by flying. If no flying speed is listed for the creature, assume a flying speed of 10 feet and perfect maneuverability. It cannot pass through solid objects, but it can move through any opening that is not airtight, even a pinhole or crack. The rules don't say how quickly a gaseous creature can cram itself through a restricted area. I recommend that you allow a gaseous creature of any size to move through a space at least 6 inches square without squeezing or losing speed no matter what the creature's size category. Allow the creature to move through a space at least 1 inch square, but less than 6 inches square as hampered movement (double movement cost, see page 163 in the Player's Handbook) and a space less than 1 inch square at a rate of 5 feet per action spent moving. I also recommend that a gaseous creature suffer no penalties for squeezing into a tight space.
A gaseous creature of any size can move through other creatures' spaces just as incorporeal creatures can (see Part Four), though they cannot give or claim cover or concealment by hiding inside a creature or object. A gaseous creature provokes an attack of opportunity from a creature whose space it enters. A creature threatens its own space, so a gaseous creature also provokes an attack of opportunity when leaving that space. Likewise other creatures can enter a gaseous creature's space; most gaseous creatures cannot make any attacks, if so, it cannot make attacks of opportunity, even if another creature enters its space. If a gaseous creature has a deflection bonus to Armor Class, a creature entering its space must make a successful touch attack to enter the space; see Part Four for details.
A gaseous creature has a Strength score, but it has no way to affect solid objects except by moving over them in a puff of air. To simulate that, try this. As a full-round action, a gaseous creature can attempt to create a wind effect in its own space. The wind speed is equal to the gaseous creature's Strength score plus the gaseous creature size modifier for grappling attacks: Colossal +16, Gargantuan +12, Huge +8, Large +4, Medium +0, Small -4, Tiny -8, Diminutive -12, Fine -16. Refer to Table 3-24 in the Dungeon Master's Guide and to the text that accompanies it for wind effects.
A gaseous creature cannot use a wind effect to handle material components for a spell or to complete somatic components for a spell. It can, however, use feats (namely Silent Spell, Still Spell, and Eschew Materials) to cast spells.
When a creature becomes gaseous, everything it was holding or wearing becomes gaseous (and nearly weightless) along with it. The rules are unclear about exactly what happens to other creatures that you might hold or carry when you become gaseous. In general, you should assume that an ability to become gaseous extends only to the user. You can make a reasonable exception for creatures you carry tucked into your clothing (or that you pick up and tuck into your clothing), and that can include a familiar, cohort, or animal companion if the creature is small enough to fit into your clothing. Of course, if a familiar, cohort, or animal companion has the share spells ability and you (the master) cast the gaseous form spell on yourself, you can share that spell with the creature.
Discerning a creature in gaseous form from natural mist requires a DC 15 Spot check; distance between the spotter and the gaseous creature increases the DC as noted in the Spot skill description.
A creature in gaseous form can attempt to hide in an area with mist and smoke, and it gains a +20 Hide bonus when doing so.
About the Author
Skip Williams keeps busy with freelance projects for several different game companies and was the Sage of Dragon Magazine for many years. Skip is a co-designer of the D&D 3rd Edition game and the chief architect of the Monster Manual. When not devising swift and cruel deaths for player characters, Skip putters in his kitchen or garden (rabbits and deer are not Skip's friends) or works on repairing and improving the century-old farmhouse that he shares with his wife, Penny, and a growing menagerie of pets.