It is sin if you are going to lose the money which is not yours, let's say that you spent your money for the tuition in your school, of course you are doomed when this thing happen because you are going to be scolded by your parents doing such thing over studying and so on, but still, gambling is a sin, a very big sin to GOD, it is written on the Holy Bible, but if you are not a religious person, then I guess that you are just going to say that gambling is not a sin as long as you are not messing with other people lives.
Gambling as per definition is a form of entertainment and by analyzing it, we can already tell we have to spend to be entertain and it's up to a certain gambler if he will seek for the money which is very hard and he will live a desperate life then that is the time he made a sin but don't blame it to gambling.