Like all Vorta field supervisors, Gelnon was the Dominion official in command of a Jem'Hadar unit.
In 2374, he commanded a Jem'Hadar attack ship during an assault on the USS Defiant while the Federation starship was surveying a subspace compression anomaly. His First, Kudak'Etan, successfully boarded and captured the ship, in the combat debut of Alpha Quadrant Jem'Hadar, the 'Alphas.' Despite his initial reservations at the abilities of the "Alphas", Gelnon expressed his satisfaction with the unit's performance.
Several weeks later, Gelnon coordinated with a member of the Orion Syndicate, named Raimus, during an attempt to destabilize the Federation Alliance. Through Raimus, Gelnon instructed Liam Bilby to assassinate the Klingon Ambassador to Farius, an opponent of the Alliance. (DS9: "Honor Among Thieves")
Gelnon was played by actor Leland Crooke. His name comes from the One Little Ship script.
The script for "Honor Among Thieves" does not name Gelnon but simply describes him as "Vorta". 
Gelnon appears as a selectable commander in the non-canon video game Star Trek: Conquest and is featured in Star Trek Online as a "Warp Theory Specialist" who can be obtained as a Klingon duty officer by completing the assignment "Zenas Expanse Renown".
Gelnon also has a card in the Star Trek Customizable Card Game.