[Kromm]

Outside of a specific setting where it's forbidden, Healing spells do work on the undead. In plain-vanilla GURPS Magic, Healing spells work on anything animate and corporeal, including not just natural living beings (regardless of IQ, biological kingdom, etc.), but also unnatural living beings (from faerie, through bioroids, down to oozing slimes), demons, elementals, undead, etc. The only things strictly excluded are entities without bodies (such as intangible spirits), artifacts (like cars, robots, and toasters), and agglomerations of inanimate matter (puddles, shed snake skins, stones, etc.).

Specific demons and undead might be exceptions to this rule. If so, that's grounds for a Divine Curse or Weakness disadvantage. For an example, see the hellhound on p. 36 of Dungeon Fantasy Monsters. Absent such a trait, Healing spells just work and do not cause harm.

As for Unhealing, that trait is very specific in its meaning: "You cannot heal naturally." Generally, Healing spells do work on Unhealing beings. The qualifier "depending on your nature" refers mainly to the "animate and corporeal" vs. "either incorporeal or inanimate" dichotomy, and to beings with specific disadvantages that cause Healing spells to not work or do harm. This is never a property of Unhealing in itself, though, which simply switches off the body's ability to recover from injury outside of specific conditions (-20 points) or altogether (-30 points).