A gambler, or gamester, was a person who engaged in the activity of gambling. Gamblers who played casino games with ridiculously high bets were known as "high rollers". Notable high rollers included Okalar at dabo. (DS9: "The Abandoned", "Badda-Bing, Badda-Bang") Someone who was often quite skilled at a game but hid their true abilities were often called hustlers. Notable hustlers included Jake Sisko at dom-jot and Kathryn Janeway at pool. (DS9: "The Abandoned"; VOY: "The Cloud")
When talking to the Providers in 2268, James T. Kirk claimed Humans prided themselves for being the most successful gamblers in the universe. (TOS: "The Gamesters of Triskelion")
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")
The job description of a Dabo girl included wearing a sexy costume and entertaining gamblers. In 2369, Sarda agreed to this, but refused to give sexual favors to Quark. (DS9: "Captive Pursuit")