gastropod vs mollusk

gastropod and mollusk both are nouns.

Word	Noun	Adjective	Verb	Adverb
gastropod	Yes	No	No	No
mollusk	Yes	No	No	No

As nouns, mollusk is a hypernym of gastropod; that is, mollusk is a word with a broader meaning than gastropod:

gastropod: a class of mollusks typically having a one-piece coiled shell and flattened muscular foot with a head bearing stalked eyes
mollusk: invertebrate having a soft unsegmented body usually enclosed in a shell

Other hypernyms of gastropod include mollusc, shellfish.

gastropod (noun)	mollusk (noun)
a class of mollusks typically having a one-piece coiled shell and flattened muscular foot with a head bearing stalked eyes	invertebrate having a soft unsegmented body usually enclosed in a shell