Old English gnæt "gnat, midge, small flying insect," earlier gneat, from Proto-Germanic *gnattaz (source also of Low German gnatte, German Gnitze); perhaps literally "biting insect" and related to gnaw.
The gnatte is a litil fflye, and hatte culex he soukeþ blood and haþ in his mouþ a pipe, as hit were a pricke. And is a-countid a-mong volatiles and greueþ slepinge men wiþ noyse & wiþ bytinge and wakeþ hem of here reste. [Bartholomew Glanville, "De proprietatibus rerum," c. 1240, translated by John of Trevisa c. 1398 ]
Gnat-catcher, insectivorous bird of the U.S. woodlands, is from 1823.
"bile, liver secretion," Old English galla (Anglian), gealla (West Saxon) "gall, bile," from Proto-Germanic *gallon "bile" (source also of Old Norse gall "gall, bile; sour drink," Old Saxon galle, Old High German galla, German Galle), from PIE root *ghel- (2) "to shine," with derivatives denoting "green, yellow," and thus "bile, gall." Informal sense of "impudence, boldness" first recorded American English 1882; but meaning "embittered spirit, rancor" is from c. 1200, from the old medicine theory of humors.
a popular name for a tiny two-winged fly, applied indiscriminately to many small insects, Old English mygg, mycg "gnat," from Proto-Germanic *mugjon (source also of Swedish mygga, Old Saxon muggia, Middle Dutch mugghe, Dutch mug, Old High German mucka, German Mücke "midge, gnat"). No certain cognates beyond Germanic, unless doubtful Armenian mun "gnat" and Albanian mize "gnat" are counted. Watkins, Klein and others suggest an imitative root used for various humming insects and a relationship to Latin musca "fly" (see mosquito). Meaning "diminutive person" is from 1796.