gangster vs yakuza

gangster and yakuza both are nouns.

Word	Noun	Adjective	Verb	Adverb
gangster	Yes	No	No	No
yakuza	Yes	No	No	No

As nouns, yakuza is a hyponym of gangster; that is, yakuza is a word with a more specific, narrower meaning than gangster:

Other hyponyms of gangster include apache, yardie.

gangster (noun)	yakuza (noun)
a criminal who is a member of gang	organized crime in Japan; an alliance of criminal organizations and illegal enterprises
	a Japanese gangster