This is what we found on Wikipedia regarding the Gareth:

Gareth (Welsh: [ˈɡarɛθ]; Old French: Guerehet, Guerrehet) is a Knight of the Round Table in Arthurian legend. He is the youngest son of King Lot and Queen Morgause, King Arthur's half-sister, thus making him Arthur's nephew, as well as brother to Gawain, Agravain and Gaheris, and either a brother or half-brother of Mordred. Gareth is particularly notable in Le Morte d'Arthur, where one of its eight books is named after and largely dedicated to him, and in which he is also known by his nickname Beaumains.