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.