I don't really believe that idea of professional gambling since gambling is a game of luck to a large extent, although sports betting has a mixture of luck and skill. If I want to force myself to accept that idea of professional gambler, I will say it's someone whose only job is gambling. As people dress up to go to work, he prepares to gamble and that is what pays his bills. He may be into other business of gambling but everything have to revolved around gambling, then we can say such a fellow is a professional gambler.
You are absolutely right bud, who ever claims to be a professional gambler and the same gambling isn't what is paying his or her bills, then such person is lying to himself and those who care enough to listen and believe his claim of being a professional gambler.
This days with alot of things people can do to make money legitimately even on the internet, I don't see why anyone would prefer to become a professional gambler knowing fully well that making money from gambling is luck dependent and there day he can go on a long losing streak which may lead him into financial issues or instabilities.
It is very important for people to choose their career wisely as wrong choices often lead to regrets in the future.