It's only a sin if you're harming yourself , your family or any body else in your environment due to your gambling activities . Religions have mixed up definitions of sins and prohibited deeds that are enlisted in their holly books . That does differ from a religion to another , but i'd prefer believing my own definition that a sin is something that we can apply the right or wrong rule on it and that it's level of danger could affect not only your present , but your future or somebody's else present and future .
Well it is not a sin if even gamble cause gambling only is a kind of pastime activities where in you were able to relax with or without pain suffering. I think it is true that gambling is sin if you are engaged in a illegal way of having gambling, and it bring bad image to you life probably, And further more a sin is when you violate what gods said to us. Thank's