I'd likely build it as a two-parter:
1. One Eye with a Mitigator limitation, since that's what any prosthetic body part that actually works is.
2. Your choice of:(a) Clairsentience with Clairvoyance, -10%, Visible, -10%, and suitable gadget limitations – definitely Breakable, -20%, but I'd keep Can Be Stolen down to the "Must be forcefully removed, won't immediately work for thief" level, for -5%. Add Anchored, -60% (borrowed from Warp) if it can't roll around and go places; Aware, +50% if you can walk around and function while the eye is out of your head; and Increased Range if it can transmit over great distances. This package is what the lovely Eden had in the dreadful Doomsday.
(b) An Ally bought as a familiar with the Telepathic lens on p. 21 of GURPS DF 5: Allies. This suits Gorgons and creepy necromancers.
For 2(a), note carefully that the net effect is that you have to take a minute to use the ability and have to physically go set it up somewhere. Anchored means Clairvoyance isn't able to look away from the physical eye while it's in your head. And in both cases, Mitigator means that while you're using the eye, you have One Eye.