gambling casino vs gambling den

gambling casino and gambling den both are nouns.

Word	Noun	Adjective	Verb	Adverb
gambling casino	Yes	No	No	No
gambling den	Yes	No	No	No

As nouns, gambling den is a hypernym of gambling casino; that is, gambling den is a word with a broader meaning than gambling casino:

gambling casino: a public building for gambling and entertainment
gambling den: a public building in which a variety of games of chance can be played (operated as a business)

Other hypernyms of gambling casino include gambling hell, gambling house, gaming house.

gambling casino (noun)	gambling den (noun)
a public building for gambling and entertainment	a public building in which a variety of games of chance can be played (operated as a business)