At some point, a powerful threat pursued him to the world Mogo after the destruction of the rest of the Green Lantern Corps. There, Ganthet escaped by transporting himself to another reality. At some point afterward, Ganthet died but had managed to protect the energy rings in his possession.
In 2262, an USS Enterprise away team to an orphan planet led to the discovery of Ganthet's body and the rings. The body was brought aboard the vessel and analyzed by Doctor Leonard McCoy who believed the ship's exobiologists would be interested in the new alien species. (TOS - The Spectrum War comic: "Issue 1")