gratuity vs fringe benefit

gratuity and fringe benefit both are nouns.

Word	Noun	Adjective	Verb	Adverb
gratuity	Yes	No	No	No
fringe benefit	Yes	No	No	No

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

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

Other hypernyms of gratuity include perk, perquisite.

gratuity (noun)	fringe benefit (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