high roller vs gambler

high roller and gambler both are nouns.

Word	Noun	Adjective	Verb	Adverb
high roller	Yes	No	No	No
gambler	Yes	No	No	No

As nouns, gambler is a hypernym of high roller; that is, gambler is a word with a broader meaning than high roller:

high roller: a gambler who wagers large sums
gambler: a person who wagers money on the outcome of games or sporting events

high roller (noun)	gambler (noun)
a gambler who wagers large sums	a person who wagers money on the outcome of games or sporting events
one who spends lavishly and ostentatiously on entertainment	someone who risks loss or injury in the hope of gain or excitement