stack up vs gather

stack up is not a noun while gather is a noun.

Word	Noun	Adjective	Verb	Adverb
stack up	No	No	Yes	No
gather	Yes	No	Yes	No

As verbs, gather is a hypernym of stack up; that is, gather is a word with a broader meaning than stack up:

Other hypernyms of stack up include collect, garner, pull together.

stack up (noun)	gather (noun)
	the act of gathering something
	sewing consisting of small folds or puckers made by pulling tight a thread in a line of stitching

stack up (verb)	gather (verb)
arrange into piles or stacks	increase in amount by collecting or gathering
	increase or develop
	conclude from evidence
	assemble or get together
	draw together into folds or puckers
	look for (food) in nature
	draw and bring closer
	collect in one place
	get people together