yakuza vs gangdom

yakuza and gangdom both are nouns.

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

As nouns, gangdom is a hypernym of yakuza; that is, gangdom is a word with a broader meaning than yakuza:

yakuza: organized crime in Japan; an alliance of criminal organizations and illegal enterprises
gangdom: underworld organizations

Other hypernyms of yakuza include gangland, organized crime.

yakuza (noun)	gangdom (noun)
organized crime in Japan; an alliance of criminal organizations and illegal enterprises	underworld organizations
a Japanese gangster