Celt vs Gaul

Celt and Gaul both are nouns.

Word	Noun	Adjective	Verb	Adverb
Celt	Yes	No	No	No
Gaul	Yes	No	No	No

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

Celt: a member of a European people who once occupied Britain and Spain and Gaul prior to Roman times
Gaul: a Celt of ancient Gaul

Other hyponyms of Celt include Gael, Briton.

Celt (noun)	Gaul (noun)
a member of a European people who once occupied Britain and Spain and Gaul prior to Roman times	an ancient region of western Europe that included what is now northern Italy and France and Belgium and part of Germany and the Netherlands
	a Celt of ancient Gaul
	a person of French descent