Very different and strange take on this topic but it feels like undeniable truth that someone should be making money to be counted true professional. Otherwise its just hobby lol. I saw no video gamers claiming they are professional unless they join as team, find a sponsor, get wage, win contest etc so on. At the point where a gambler can generate money through he can be called pro. I think we may count sponsorships too - gambler doesn't need to be always making money only through gambling he is mastering.
I think you can still be called as a professional even if you are not earning with your skill. To earn is only a matter of choice but what values mostly by the professional is their talent. They are called professional because they are really good at what they are doing. I am not really sure if hobby is different from it but I know that you can also make money out of your hobby.
It's a great way to make a living this way because you enjoy what you are doing and you won't feel like you are working. A professional gambler can also have other talents and they can choose to make money on those. They can also apply for a real job if they want to. To have multiple income streams is always better, as it provides us more security.