horse fly vs gadfly

horse fly and gadfly both are nouns.

Word	Noun	Adjective	Verb	Adverb
horse fly	Yes	No	No	No
gadfly	Yes	No	No	No

As nouns, gadfly is a hypernym of horse fly; that is, gadfly is a word with a broader meaning than horse fly:

horse fly: large swift fly the female of which sucks blood of various animals
gadfly: any of various large flies that annoy livestock

horse fly (noun)	gadfly (noun)
large swift fly the female of which sucks blood of various animals	any of various large flies that annoy livestock
	a persistently annoying person