gang vs gang up

gang is a noun but gang up is not a noun.

Word	Noun	Adjective	Verb	Adverb
gang	Yes	No	Yes	No
gang up	No	No	Yes	No

As verbs, gang and gang up are synonyms defined as:

gang (noun)	gang up (noun)
tool consisting of a combination of implements arranged to work together
an organized group of workmen
an association of criminals
an informal body of friends

gang (verb)	gang up (verb)
act as an organized group	act as an organized group