Gambling

This is a reference issue, and it begins with the Gemara. The argument is that when a person approaches a bet, he is betting that he will not lose and therefore does not have the will to pay if he loses. This depends on all sorts of differences and opinions. Although according to the law, if it is clear that there is such a will, it is valid. This is a question of fact (whether or not there is a will), and in this case we do not follow the Gemara and the poskim, but rather an assessment of reality. There is another issue, and it is that engaging in this expresses that you are an idler who does not engage in the ways of the land (and this disqualifies you from testifying). But this is only if it occupies you for a significant part of your time.