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 .
If both time and money are being controlled correctly it can treat as a hobby, though if the purpose is making money and being out of control then it might trigger addiction and may harm the person both financial and mental, more on self assessment and explanation if how they spend time and how they spend their finances, just like taking a movie night every sunday with budget that willing to spent, if there's no excess when playing gambling then can qualify as a hobby.
Well that's it, as you said that something can be called a hobby when the activity is accompanied by full management and caution, this is gambling which is an activity that involves money, opportunities and risks, it's okay to make gambling a hobby, but when it turns out that the choice actually causes a lot of problems in your life then it is clear that it should be avoided, I think we should be able to choose something right to be a hobby, and if you are not too interested in gambling then it is clearly better to choose something else to be your hobby. Basically and in essence it is okay to make gambling a hobby, but in any case this is a choice that is too vulnerable or too dangerous.