Gambling is a form of entertainment, and is not, in itself, sinful. There wouldn’t be Bingo games and raffle tickets at Catholic parishes if it were.
The Catechism says
2413Games of chance(card games, etc.) orwagersare not in themselves contrary to justice. They become morally unacceptable when they deprive someone of what is necessary to provide for his needs and those of others. The passion for gambling risks becoming an enslavement. Unfair wagers and cheating at games constitute grave matter, unless the damage inflicted is so slight that the one who suffers it cannot reasonably consider it significant.
I enjoy Blackjack. And sometimes just sitting at a slot machine getting hypnotized. I can be in Vegas in 4 hours, by car. But I only gamble a couple times a year, when we’re passing through Vegas on our way to somewhere else. I figure out what I can afford to lose, bring the cash, and if I lose it all, that’s it. I don’t go to card rooms, Indian casinos (There are 3 or 4 within an hour or so of me),or the horse races; I don’t buy lottery tickets or scratchers, don’t enter the football pools at work, and so on. It’s not a problem for me.
If someone starts gambling money they need to pay rent, deprive their family of the necessities of life, all the stuff compulsive gamblers do, then it’s a problem. Then they are sinning. Not because they are gambling - as said before, it’s just entertainment - but because they’re being irresponsible.