sum vs nitty-gritty

sum and nitty-gritty both are nouns.

Word	Noun	Adjective	Verb	Adverb
sum	Yes	No	Yes	No
nitty-gritty	Yes	No	No	No

As nouns, sum and nitty-gritty are synonyms defined as:

sum and nitty-gritty: the choicest or most essential or most vital part of some idea or experience

sum (noun)	nitty-gritty (noun)
the whole amount	the choicest or most essential or most vital part of some idea or experience
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

sum (verb)	nitty-gritty (verb)
determine the sum of
be a summary of