biting midge vs gnat

biting midge and gnat both are nouns.

Word	Noun	Adjective	Verb	Adverb
biting midge	Yes	No	No	No
gnat	Yes	No	No	No

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

biting midge: minute two-winged insect that sucks the blood of mammals and birds and other insects
gnat: any of various small biting flies: midges; biting midges; black flies; sand flies

biting midge (noun)	gnat (noun)
minute two-winged insect that sucks the blood of mammals and birds and other insects	any of various small biting flies: midges; biting midges; black flies; sand flies
	(British usage) mosquito