Professional gamblers, or in other words, people who try to make gambling a source of income.
This isn't easy, as gambling isn't something you can calculate your potential returns on and should only be considered for fun.
If you get too involved, you might not only lose money but also become bankrupt or addicted.
Gambling is indeed fun and can result in big wins, but it is still not recommended to make it like a job to earn income.
If we have skill/knowledge in specific game especially poker and sports betting, making gambling as source of income is fine.
Afaik there are many pro poker players and pro sports bettor who can make a living from gambling but of course they do it wisely and responsinbly.
However, if we are talking about luck based game where we play against the house, it is no recommended at all to make it as a source of income.