Summary: A gnat is the informal name for many small insects of the order Diptera. The gnat category often includes midges, sand flies, small flies, no-see-ums and even mosquitoes.
The ubiquitous gnat seems to be found in all the places where they are least welcome. We certainly do not begrudge the gnat for flitting around our heads when we are sitting out by a pool. I mean, they need to call somewhere “home”. We also expect to find them if we are walking in the woods. After all, they were there first. Right? But, where we like them least is in places we claim as our own like inside our homes or places we go to dine.
So, what is a gnat? And, it is gnat, not knat or nat. Drives me crazy when I get a question about knats. I don't even know what a knat is, let alone trying to get rid of one. Sorry! Anyway, a gnat is simply a term that was invented to cover a bunch of tiny flying insects that otherwise have long Latin names like Tipulidae, Simuliidae, or Sylvicola fenestralis. That said, there are a few very specific flying insects that have specific gnat-names like the Wood gnat or Window gnat, which at a glance, by the way, look a lot like all the other gnats I have seen. The majority of insects falling under the gnat title are biters. They take a blood meal when and where they are able. They are rapid breeders and are able to build enormous populations in a very short time period.
Most people visiting this site neither care about how you spell it or how to properly introduce it by its formal name. Nearly everyone wants to know how to get rid of the darned things. Generally speaking, you do not get rid of gnats. They live around water sources as a rule, unless you are visiting a dessert where you can be eaten alive by sand fleas, sand flies or gnats. Again, call them what you like.
The best defense against gnats is a good offense. Eliminate standing water. Cut down high grass and weeds. Do not over-water your plant materials or gardens. Nearly any insecticide will kill gnats, but will not prevent them from returning if you have not eliminated their breeding ground. Treating standing water with Bti (Bacillus thuringiensis var. israelensis) is one method of control, but be sure to read the label for the right type of Bti. Some are only for mosquito control, so you need to know what your target pest is before you plunk down your cash. Numerous products with Bti are available on line.
If you love the outdoors, but hate the nightly gnat attack, turn to good old DEET products. The stuff works! One little word of caution about DEET. The stuff was originally used as a pesticide on farm fields. The military stated using it in 1946 and it became available to civilian populations in 1957. You should not apply DEET under clothing or on cuts or burns. It is comforting to know that the seizure rate associated with DEET is only about one in 100 million users.
Don't care to be part of a statistic? Give some of the natural alternatives a try. Eucalyptus-based repellents will leave you with a distinctive odor, but gnats will leave you pretty much alone. Citronella oil is another odorous alternative. The most interesting one I found is distilled nepetalactone. That's the active ingredient in catnip. It repels mosquitoes ten times better than DEET. Instead of gnats, you'll have to content with cats.