Being a professional does not mean that a gambler always wins or is always lucky with every bet. The same goes for professional traders; they do not always profit either. However, they do have self-control and good experience to minimize the risk of larger losses. There is no exact measure for you to be considered a professional gambler. Although you still face risks and rely on luck in betting, you can become wiser in managing your bets and build your own professionalism.
You are right mate, professionalism means that someone has an explicit knowledge about something and they can do that thing perfectly since they knows about the ins and outs of that thing. I consider myself a professional in gambling not because am successful in it but because I can no more get addicted no matter how I get reckless at times and because it's and activity of luck, I respect it that winning is not regular. Trading can be a pin on the ass too but but if someone becomes a professional in it, they should know how to take calculative risk and minimize losses or prevent a huge lose.