In 1st edition AD&D, a Ring of Regeneration will heal its wearer's wounds, even past death (bringing you back to life if you pass a System Shock roll). It doesn't matter when you suffer an injury.
By 3rd edition, though, it only healed you while you were alive. And, not only that, but:
" [ . . . ] only damage taken while wearing the ring is regenerated."
- D20 SRD
Clearly the Wizards Guild had become an HMO - no pre-existing conditions are affected by the ring!
Making one of these rings in GURPS is pretty simple, though.
Assuming you want it as a gadget (bought with character points, say for a GURPS Supers game), the old-school style is some level of Regeneration (usually Regular - 1 HP per hour, a little slower than the D&D one, possibly Fast - 1 HP per minute, much faster than the D&D one) plus Regrowth and Unkillable 1, all of which affect whoever has the ring. The new-school style drops Unkillable 1, and adds an Accessibility limitation to "Only wounds suffered while wearing the ring."
As a magic item, it's the same, just don't bother with the point cost unless you need to cost it out for sale or as a Power Item for DF. Personally, I'd just wing it.
And for no special reason, I'm going to link to my favorite OOTS reference to a Ring of Regeneration.