noun
A person who makes a living from gambling or is habitually engaged in betting games.
彼は若い頃、賭け事師として生計を立てていた。
When he was young, he made a living as a gambler.
ギャンブラー is a loanword from English and carries a modern, sometimes stylish connotation, whereas 賭け事師 is a native Japanese compound with a slightly more old-fashioned or matter-of-fact feel.
Compound of 賭け事 (gambling, betting) + 師 (master, specialist). The exact historical derivation is uncertain, but the word follows a common pattern for occupational nouns.