Gambling in prison is more of a kind of entertainment for them. Since no one would gamble for money, prisoners would only gamble for what’s only available to them like their personal thing, foods or anything that can be gamble and would bring excitement to both of the players.
However, gambling with money is strictly prohibited so their kind of gambling is different from gambling in the real casino. But even though, if their gambling activities suddenly lead to a fight and mess up with other prisoners, still it will never be allowed inside the jail.
I dont believe that they gamble for entertainment. It does not look that people in prison are people who search for entertainment or fun in life, or they get that from something else. As gambling for money is forbidden, it does not mean they dont gamble for money. All the money lost and won can be written down on paper (like virtual money), and later asked for debt when a person gets released or ask to find way to give lost money to his people outside jail (and it non of his business how a person who has lost is going to do that).