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.
Professionals and amateurs have always been ambiguous. Do professionals get paid? Is it the depth of knowledge, years of practice, and specialization?
For instance: A Hacksaw slots player with a natural understanding of game mechanics, strategy, and probability variables. Though not a professional gambler, this person may be better than those who do.
Its apparent that 'professional' has numerous meanings. Professionalism may be best viewed as a spectrum with monetary reward as one of several markers. Multiple income streams enrich skills and finances.