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.
Maybe other games such as sports is possible in prison.
There might some spots that are not monitored by CCTV and there will be ways for them to gamble. But you might be right as well that it's just all about the TV series' that we see that the prisoners are doing things like gambling because that's how the masses portrait their thoughts on them.
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.
I'll never think that I'd ever get into it even before I think of gambling there.