There are four ways to be immune to Mind Control spells (and specifically those . . . I'm talking about spell-based, mana-dependent magic only, not psi, weird tech, or whatever):
1. Be nonsapient (racial IQ 0-5).
2. Lack free will (have the Automaton meta-trait or anything similar).
3. Belong to a group that has a specific spell for controlling it.
All of those points are made on p. 133 of
GURPS Magic. Absent one of those:
4. Have Immunity to Mind Control. This is explicitly added to many monsters that are sapient and free-willed, and for which there's no clear control spell. It's sometimes given – albeit superfluously – to creatures that would be immune for one of the three previous reasons.
—
There are two ways to be immune to
all Body Control spells:
1. Lack a body (like any incorporeal or virtual being). This is a common-sense consideration.
2. Have Immunity to Body Control. That's no less legitimate than Immunity to Mind control.
In addition, not being alive is general protection against all Body Control spells except those noted as specifically affecting classes of beings that aren't alive (e.g., Deathtouch).
Finally, lacking the specific capacity targeted by a spell can grant immunity
to that spell. This means there's no point in casting Choke on a target with Doesn't Breathe; Hunger, Nauseate, Retch, or Thirst on a being with Doesn't Eat or Drink; Rooted Feet on something with No Legs (Aerial); Strike Blind on a subject with Blindness; Weaken Blood on a creature with Injury Tolerance (Diffuse or No Blood); etc.
On that last point, Immunity to Metabolic Hazards amounts to blanket immunity to disease, poison, environmentally induced illness, and vague "sickness," so it will block Nauseate and Retch, for instance, but
not Choke if the subject somehow has that advantage yet must breathe, Hunger if the subject somehow has that advantage yet must eat, and so on. A common source of confusion is write-ups for monsters with this advantage that lump the immunities it grants in with immunities granted by other traits, like Doesn't Breathe and Doesn't Eat or Drink.