GURPS Martial Arts introduces this on page 137.

Kromm in 2013 wrote about targeting the CERVICAL (neck) spine in particular:

By a strict reading, if the spine in the torso (0) gives -8, then spine in the neck (-5) should give -13. I find this rather excessive, though. I would simply observe that the usual ratio of cervical vertebrae to the remainder of the spine is about 1:4, and note that 1/4 has the effect of adding between -3 and -4 to hit on p. B550. I'd go with -3 and call this -11 to hit.

With this end result:

Cervical Vertebrae (-11): Crushing, cutting, impaling, piercing, and tight-beam burning attacks from behind can target the spine in the neck. The vertebrae provide an additional DR 3. Use the wounding modifiers for the neck, but any hit for enough injury to inflict a shock penalty requires a knockdown roll, at -5 if a major wound. Injury in excess of HP cripples the spine. This causes automatic knockdown and stunning, plus all the effects of Quadriplegic (p. B150). Roll after the fight to avoid gaining this disadvantage on a lasting or permanent basis! A miss by 1 hits the neck.