The name first makes an appearance in Sir. Thomas Le Mort D’Arthur (c.1485) as the name of one of the knights of the Round Table. Malory is said to have based the name on Gahariet, which is the name of a knight in the French version of the King Arthur mythos. It is believed by most sources that the name has a Welsh origin and might be related to Geraint (a Welsh form of the Greek Gerontius meaning “old man); or the Welsh word gwaredd (gentleness; kindness). Other sources have connected it with the Welsh word gwyhrt (worth; value), and it has also been suggested to be Franconized form of the Welsh male name, Gwoerydd (Lord of grass; Lord of Hay). The name may not have any Welsh origins at all, the name Garret/Garet have appeared in use in France and Spain since the 7th-century, both names are an early diminutive form of Gerald or Gerard.
In England and Wales, the name appeared in the Top 500 between 1996 and 2005, peaking at #117 in 1996. In Northern Ireland, Gareth cracked into the Top 100 between 2001 and 2003, peaking at #87 in 2001.
Short forms are Gary and Gaz.