If I ever get into prison, gambling will be the last thing I would do there. I will do my best to find a less stressful time killer and entertainment.
This prison gambling usually just fictional since they are well monitored on the CCTV by police while they are not allowed to hold money and other valuable things in the prison most importantly any gambling materials.
Most of the prisoners do task depending on the country. So it’s very rare for prisoners to have allowed time to gamble among prisoners.
Prison in your country must be totally different from prisons in Europe or Post Soviet countries. Prison in those countries dont have CCTV in cells, and cells are not for one or two persons. It usually a room with one small windows with bars and 10-20+ prisoners in there. Metal door with a small windows separates cell from hall. Prisoners are not allowed to gamble or have accessories for gambling, but they do manage to create cards, backgammon or chessboard in cell from materials they have. As they spend majority of time in cell, they have nothing to do. I suppose you talk about prison where prisoners work, but they do it only for several hours a day (that is where they steal materials for crafting btw). I dont think that prisons are so well monitored as you imagine. Maybe your vision about prisons are built from movies, tv or in your country prison is a new and fancy building. In Europe, its usually already old buildings, where guards patrol halls once in a while, and maybe few times a day step in cell to look around.