Based on professional meaning you get paid based on your skill, in gambling if you can win more and less loss I think you must called as professional gamble. But if your gambling statistic less win but more loss seems can't called as professional. The same way with professional football they can earn much money from football skill and get bigger payment reward in contract with the club, if include in the gambling you have expert skill in gambling what ever kinds of betting and most important you earn income from your betting skill.
No matter how long you are in gambling if statistic less win than more losses you still amateur of gambling, but earn consistent income from gambling more win than less losses your a professional gambler.