Impermanent, Dispelled by Iron is a -20% limitation applied to Magery on page 108 of GURPS Fantasy.
As page 68 mentions under Faerie Glamour:
- Faeries thus have their own special form of magic, based on Impermanent Magery (p. 129).
129 mentions that -10 becomes -20 when there is "some nonmagical means of dispelling the effects"