It depends on the gambler how he treats gambling. There are others who play for fun, so with this type of gambler, we can assume that gambling is a hobby for him, right?
But for a gambler whose aim is to make money, he/she cannot say that gambling is his hobby, especially if he already has an addiction to gambling that we know his body is looking for every day of his life.
If the player is trying to make a game with his work and it is unlikely that it will already be a hobby, but rather the main way of the ease. And a fool who really play for the sake of entertainment on weekends or with friends in some kind of short period can definitely be called a hobby in this case .
In other words, if the hobby is not getting you distracted or harming you in any way such as financially then we can consider it as a hobby.
If someone is gambling casually then he is having gambling as a hobby but if someone is repeatedly gambling and losing money which he cannot afford to lose then it's not a hobby but an addiction.