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.