The insect normally referred to as a gnat is actually a small mosquito – correct name: Culex pipiens. It is common in gardens on warm evenings. Another species, the true Window Gnat, is a slow-flying insect about 8mm long with wings more rounded than those of the mosquito.
The Window Gnat tends to stay on windows and is therefore easily killed by a good swat, or by spraying with a fly killer aerosol.
For a heavy Gnat infestation, we would recommend you contact professional pest control company to administer a professional treatment to solve the problem. A trained professional will have the technical knowledge and access to a range of professional use products and equipment which are not available to the public.