mnemonic vs gimmick

mnemonic and gimmick both are nouns.

Word	Noun	Adjective	Verb	Adverb
mnemonic	Yes	Yes	No	No
gimmick	Yes	No	No	No

As nouns, gimmick is a hypernym of mnemonic; that is, gimmick is a word with a broader meaning than mnemonic:

Other hypernyms of mnemonic include device, twist.

mnemonic (noun)	gimmick (noun)
a device (such as a rhyme or acronym) used to aid recall	any clever maneuver
	something unspecified whose name is either forgotten or not known
	a drawback or difficulty that is not readily evident

mnemonic (adjective)	gimmick (adjective)
of or relating to or involved the practice of aiding the memory