black fly vs gnat

black fly and gnat both are nouns.

Word	Noun	Adjective	Verb	Adverb
black fly	Yes	No	No	No
gnat	Yes	No	No	No

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

black fly: small blackish stout-bodied biting fly having aquatic larvae; sucks the blood of birds as well as humans and other mammals
gnat: any of various small biting flies: midges; biting midges; black flies; sand flies

black fly (noun)	gnat (noun)
small blackish stout-bodied biting fly having aquatic larvae; sucks the blood of birds as well as humans and other mammals	any of various small biting flies: midges; biting midges; black flies; sand flies
	(British usage) mosquito