Professional has been defined differently by various individuals and organisations
A professional gambler is someone who has complete control over the time, money and energy they spend on gambling
A professional gambler, by definition, is someone who makes a living off of their gambling winnings.
https://www.algamus.org/blog/thinking-you-are-a-professional-gamblerThe first definition sees a professional gambler as one who has total control over his gambling activities. When a gambler ensures that his habits don't have any negative effect on his finances, work, family, and other endeavours, it means he is a professional.
The second definition relates professional gambling to financial gains. The set of people who have been able to depend on gambling as a source of income are the professionals.