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 .
As long as we are not harming our self or anyone around us, playing gambling is not a sin. I strongly believe that it is not a sin but people who plays gambling make it looks like a sin. Their addiction in gambling has made them change and sometimes the change is just too much that their addiction had lead them to do bad things. Other countries seriously take gambling as a sin but some has treat as a source of income in which some people by playing relays this happiness/