gnat vs biting midge

gnat and biting midge both are nouns.

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

As nouns, biting midge is a hyponym of gnat; that is, biting midge is a word with a more specific, narrower meaning than gnat:

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

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