According to How Stuff Work, gnat bites are treated by washing the affected area with soap and water. One must avoid scratching the itchy area. The itching and swelling can be treated by applying ice or cold metal objects or by taking over-the-counter medication, such as antihistamines or NSAIDs.
U.K. National Health Services and WebMD mention that gnat bites cause itchy skin lumps, and the area often becomes red and inflamed. The hole of the sting remains visible, and fluid-filled vesicles can develop for more sensitive people. Mosquito bites do not usually pose a health risk, however there is the possibility of a severe allergic reaction.