The only way I can imagine gambling on a professional level is if you're the one running the casino. Otherwise, I can't imagine a player as a professional or considering it a real job. Games are primarily based on probability and luck; it's mathematically impossible to calculate monthly or annual income. You could win a huge amount, or you could be left penniless.
It's not my idea, perhaps because I've seen gambling as a pastime or a way to gamble with luck.
I don't mean to discredit anyone with this statement, but this is my point of view.
The only people that can be professionals in gambling are casino owners because it's their business and the professionals that works in casinos because they are sure of receiving their salaries. But for the gambler who plays games and bet in casinos it is not profitable for them to be professionals because their strategies and analysis are not a guarantee to give them wins. You cannot be a professional in a job that luck is a major factor that determines wins or profit. Gambling should be where people go to entertain themselves with amount that they can afford to loose.
I think that's obvious though, since you are the owner of the casinos then you are the one going to profit from gamblers themselves. But we have seen poker players turning into professional players/gamblers.
But not all of them thrive to become successful though. And just like in professional sports only few names stand out. With that, if you really wanted to be a elite and professional player then you really have to master everything. So it's not as easy as it sounds. Those players that we watch and admire could have gone from scratch but they continue because this is want they wanted in life.