Gul Russol was a Cardassian officer who was one of security chief Odo's most reliable informants. Russol was executed by Central Command in 2374, when the Cardassian Union joined the Dominion.
In early 2375, Weyoun 6 sent a coded message to Odo supposedly from Russol, asking to meet with him on a moon in Cardassian space. It was there that Odo found out that Russol was dead. (DS9: "Treachery, Faith and the Great River")
This character was only mentioned in dialogue.
The character also appears in Dawn of the Eagles, the last book in the Terok Nor trilogy, where he is given the first name Gaten.