I've met people who've been to prison. One of the simplest gambling games they use is a game made with paper balls (often toilet paper).
They basically create balls, three for each person, then throw them at a wall. To win the game, the balls must land as close to the wall as possible. The person with the highest score wins.
They usually gamble on cigarettes, but sometimes they'll also gamble on a pair of shoes or meals.
It's amazing how people invent everything; the human mind is always amazing.
I've heard that people in prison never gamble for fun or to kill time. They gamble to have other player be in debt with them, to make him ask his relatives outside to pay or do to something for other prisoner favor. Also cheating in gambling is very popular in prison. From stories, every person who offer you to gamble with him (play cards for example) is a professional gambling cheater and its impossible to win.