For me yes, it's a sin to gamble because every gambler has the attitude of being greedy. Because  every gambler wants to win the game and even if you win you want all or plenty money  will you get. While your winning you're greedy to win more, want more or get all. Gambling is a part of being greedy because money of others in the gambling site is you want to get it. It's like you want to get what others who owned already. Because money in gambling is money of all people loser. That's all I know why gambling is sin because you focus on the money of losers and you want to have it.