It is not a sin, it will only be a sin if you do bad things or against the law of God while gambling. Say for example, you lost big and you don't have money anymore to gamble, since you are desperate to gamble for a chance to win back your losses you rubbed a bank to have a money for gambling then I would say that is a sin.
We are sinners that cause Jesus to be sacrifice from the cross to save us but a person tend to sin and I think all sorts of material things in these earth are all sins because you let it consumes you so much and I think that includes gambling and even if Jesus pays for our sin we would easy make sins every time we cling so much from all earthly things ( now I am preaching hallelujah )
And gambling is not always that good so that have to change and you must realize that gambling is just made for fun.