From BiblePortal Wikipedia

Webster's Dictionary [1]

(1): ( n.) Any fly resembling a Culex in form or habits; esp., in America, a small biting fly of the genus Simulium and allies, as the buffalo gnat, the black fly, etc.

(2): ( n.) A blood-sucking dipterous fly, of the genus Culex, undergoing a metamorphosis in water. The females have a proboscis armed with needlelike organs for penetrating the skin of animals. These are wanting in the males. In America they are generally called mosquitoes. See Mosquito.

Wilson's Dictionary of Bible Types [2]

 Matthew 23:24 (a) The Lord Jesus used this insect as a type of the estimation we place upon sins or unpleasant things in our own lives; or little things in the lives of others to which we do not object. This is in comparison with the big animal, the camel, which represents the same kind of things in the lives of others, which we do not condone.

Smith's Bible Dictionary [3]

Gnat. A species of mosquito mentioned only in the proverbial expression used by our Saviour in  Matthew 23:21.