grand total vs sum

grand total and sum both are nouns.

Word	Noun	Adjective	Verb	Adverb
grand total	Yes	No	No	No
sum	Yes	No	Yes	No

As nouns, sum is a hypernym of grand total; that is, sum is a word with a broader meaning than grand total:

Other hypernyms of grand total include amount, total.

grand total (noun)	sum (noun)
the sum of the sums of several groups of numbers	the whole amount
	the choicest or most essential or most vital part of some idea or experience
	a quantity obtained by the addition of a group of numbers
	the final aggregate
	a set containing all and only the members of two or more given sets
	a quantity of money

grand total (verb)	sum (verb)
	determine the sum of
	be a summary of