The Bible does not say 'Thou shalt not gamble.' That being said, in general, gambling, if it is financially 'material' is a waste of money. Playing poker for pennies probably isn't going to hurt anyone. Gambling can also be associated with drinking also in some contexts.
I don't see where scripture teaches, 'Thou shalt not gamble.' I think we should discourage wasting money on slot machines and lottery cards. Can I say that God will never lead anyone to gamble for a specific purpose? I cannot say that, but just based on wisdom, it would seem a good thing to avoid. For some people, gambling triggers an addiction.
There is also the ethical issue that if you win, you could be taking the diaper money from another addict (e.g. in poker) and you are not doing something productive to earn it. Gambling doesn't do much to benefit society. Some people's jobs, also, aren't all that helpful to society.
Pool and playing cards are used for gambling. There are also some people who are against raffles. If you pay to play Bingo and have a chance to win prizes for fund-raising, isn't that technically gambling? (Is that how they do fund raising with Bingo?) I don't see a strong Biblical case for the idea that this is sinful.
There have also been Christians who are against 'worldly entertainments.' A lot of early Christians didn't go to the dramas, but they were pagan, with false gods and such. There were sporting events dedicated to other gods.
Post-Reformation, there were Protestants, for example among the Puritans, who seemed down on merriment, or certain types of it. In the Holiness movement and among many Pentecostals before the 1960's or 1970's, some Christians were against going to the 'movie house'. Some were against going to sporting events. I don't see a good case for this in scripture. (Of course, there are still Olympics, with images of other gods at the opening ceremony, at least arguably commemorating other gods, which may be a moral issue.) Some people get too obsessed with sports and waste too much of their time on entertainment. It's a matter of stewardship of time, in most cases, not 'thou shalt not.'