Klingon Empire emblem insignia image.
Gelt was a Klingon who visited Argelius with a group of fellow Klingons in 2270. While patronizing a cantina in Yelgor City, the Klingons got into a brawl with Lieutenant Commander Piper and her friends when Gelt harrassed Piper while she was performing a dance. The brawl ended when Captain James T. Kirk and Commander Spock intervened. Gelt and his group later made their way to the scientists' compound where the transwarp device had been developed. When the Klingons found Piper and her group at the compound, Gelt hoped both to seize transwarp and get his revenge on Piper, but the Vulcan scientist Perren, hidden in an alcove, stunned some of the Klingons with a phaser, allowing Piper to knock Gelt out with an air-conditioning unit. It was presumably Gelt who subsequently informed the Klingon Empire that the transwarp device was aboard the USS Enterprise. (TOS - Fortunes of War novel: Battlestations!)