Where do gnats live?

Yes, gnats [of the two winged suborder Nematocera] are herbivorous. They largely feed off plants, which they may damage considerably in the process. But there are some that also are carnivorous.

Gnats belong to the two winged order Diptera within the class Insecta. Within that order, they're members of the Nematocera suborder. That means that they're related most closely to mosquitoes [Culicidae], crane flies [Tipulidae], and black flies [Simuliidae].