[Varyon]

I’m not certain what the right way to build a phylactery in GURPS is. For the “dead if destroyed” bit, you’d have an Instant Death Disadvantage worth somewhere between [-100] (from Terminally Ill, less than one month) and [-300] (as implied by the value of Heart Attack on Affliction), negated by an appropriately-limited Advantage.

As for the Gadget Limitation, there’s the issue that Gadget is normally for items you keep with you, not “so long as this item exists somewhere*, I have this ability.” The latter opens up a lot more options for protecting the item, making it much less limited than a typical breakable gadget; that indicates less of a discount, but I don’t know how much less is appropriate.

*Not necessarily even in the same plane; in Order of the Stick, Xykon’s phylactery is currently hidden in a fortress in the Astral Plane.