Is gambling a sin?

Yes, it is a sin, and first of all because the Church says so, not just because I say so. The reason is that you are throwing away money you have no guarantee of getting back; the odds in every casino are stacked in favor of the house, so by sheer statistical logic they are going to win, which is why the states license them. Even the roulette wheel has that green space that tips the odds from fifty-fifty to about fifty-one to forty-nine, and over enough spins that small margin makes the casino millions while you lose. Once you know the game is rigged against you, you really have no business walking in, and if you somehow start winning consistently they will politely ask you to leave. The same logic applies to the stock market unless you devote your life to it and become a professional; otherwise you are simply placing a bet that can go up one day and down the next. The world likes to take these risks because it has nothing else to live for, but we have Christ and the coming kingdom, so if you want to place a bet, bet on getting people into the kingdom of God, because we each have only one chance, as Hebrews 9:27 says, to face death and then the judgment.