gratuity vs perk

gratuity and perk both are nouns.

Word	Noun	Adjective	Verb	Adverb
gratuity	Yes	No	No	No
perk	Yes	No	Yes	No

As nouns, perk is a hypernym of gratuity; that is, perk is a word with a broader meaning than gratuity:

gratuity: a relatively small amount of money given for services rendered (as by a waiter)
perk: an incidental benefit awarded for certain types of employment (especially if it is regarded as a right)

Other hypernyms of gratuity include fringe benefit, perquisite.

gratuity (noun)	perk (noun)
a relatively small amount of money given for services rendered (as by a waiter)	an incidental benefit awarded for certain types of employment (especially if it is regarded as a right)
an award (as for meritorious service) given without claim or obligation

gratuity (verb)	perk (verb)
	gain or regain energy