comedian vs gagman

comedian and gagman both are nouns.

Word	Noun	Adjective	Verb	Adverb
comedian	Yes	No	No	No
gagman	Yes	No	No	No

As nouns, gagman is a hyponym of comedian; that is, gagman is a word with a more specific, narrower meaning than comedian:

comedian: a professional performer who tells jokes and performs comical acts
gagman: a comedian who uses gags

comedian (noun)	gagman (noun)
a professional performer who tells jokes and performs comical acts	someone who writes comic material for public performers
an actor in a comedy	a comedian who uses gags