A gambler, or gamester, was a person who engaged in the activity of gambling.
In 2365, Data described the story of Hotel Royale as "the story of a group of compulsive gamblers caught up in a web of crime, corruption and deceit". In this novel, both Texas and Vanessa were gamblers. (TNG: "The Royale")
In 2369, Odo called Quark a gambler and a thief. Quark refused the term thief, but accepted being called a gambler, which was affirmed by Benjamin Sisko. Later that year, Quark called himself a gambler when discussing with Odo how to deal with the Wadi. (DS9: "Emissary", "Move Along Home")