I wouldn't say that gambling definitely affects studies so much, particularly in cases where they gamble responsibly.
Btw. I'm not saying it don't affects studies either. Specially in cases where students create debt in order to cover expenses or become gambling addicts. I'm just saying it depends on person.
OP has posted that there are students who gamble during studies, or use money they suppose to pay for studies for gambling. I dont know what exactly OP wanted to discuss with topic name like that, when what he has posted is a direct indicator how gambling affect studies. Anyway, the general answer should be: if someone gamble when he needs to study, of course it will make affect on result; if someone gamble during his free time, it is up to him how to use that time and it has only tiny affect on studies (for example a person might be in bad mood after loss, or will think about his loss instead of focusing on studying).