Are you a Christian? A muslim? If not, I don't think it is a sinful thing. Or is gambling in your country? If still no, why are you sinful? It is real life.
I think these religions consider gambling as a sin because of the reason of addiction, people often become addict to gambling and they do not care about their wealth and their future and continue to gamble with them. So I think in those games where we do not become addicted and play there only for getting some entertainment it will be fair.