If you are a addicted to gamble, if your addiction destroys yourself, if your family is affected by your addiction, if you are frustrated about your addiction then definitely gambling is a sin.. Every negative addiction is a sin..
So it depends on your level of addiction identify that whether it is sin or not...
Actually the level of gambling addiction can ultimately be controlled by getting positive feedback if well thought out, most gamblers get discouraged into the hole so the definition of gambling is a very wide open crime, there is a fine line between fun and addiction.
If in the real life of pleasure culminating in the imposition of a very bad desire can be neutralized on the basis of religion, in gambling also applies equally in more logical way, there needs to be a real understanding to make changes in every error in gambling so that gambling is able to generate positive feelings .