Ive been to jail and they let me out same day as it was false charges, not sure that counts. They should have least given me free meal or maybe best they didnt :p
You will never stop gambling its part of the human spirit but I would avoid it in a jail as winning will make you unpopular with others, also your winnings makes you a target immediately surely. The best gamble is risking losing some item you didnt want or need worst case and gaining something you got to have like a phonecall perhaps, maybe that makes sense.
Life in jail is definitely far different from life in the real world, more like they are living a life that is completely opposite upside down of the life we live in the real world, and thank goodness your being to jail wasn't as a result of you commiting any crime but was as a result of fault charges, and thank goodness that you didnt spend up to a day there, for I don't know the country you are from and how the prisons over there is..
Here in my country, hearing stories of people in prison, or even going for a visitation will make you pray never to commit any crime that will land you there(in prison), you also will not even wish for their free meal for that also is poison.
And i agree with your opinion as regards gambling in prison, it's a place where only the strong and the fittess survives, gambling with the intention of winning in such places simply means that you are a strong person who can withstand any form of attack from other inmates who might not be happy with you winning something they themselves had special interest in.