English gnat is from Old English gnaet, akin to gnidan, to rub, related to Modern English gnaw and its Old English predecessors (Partridge, Origins--a Short Etymological Dictionary of Modern English). Thus it seems a gnat originally referred to a small biting fly.
Identification
The most distinctive identification feature is the antennae which have 6 or more segments. Most have long and slender antennae, which in some families can be highly plumose. Other families have short, thick antennae (eg. the march flies - Bibionidae). Most of the flies in this group have slender bodies and long, narrow wings.
Remarks
This group contains the most "primitive" fly families, and is paraphyletic to the rest of the flies. It contains the very diverse crane fly families (Tipuloidea 1,2,3), along with several families of blood-feeding flies, such as the mosquitoes (Culicidae), black flies (Simuliidae), and no-see-ums (Ceratopogonidae). Infraorder arrangement based on Tree of Life.
