[note 1] 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.

As the protagonist of the story's final episode, he slays the giant known as "Little Knight", thus avenging the death of fairy king Brangemuer, son of Guingamuer and the fay Brangepart.

For this reason, he arrives at Camelot in disguise as a kitchen boy as le bel inconnu, or the Fair Unknown, who comes without a name and therefore without a past.

Hearing this, Gareth decides to spare the Red Knight, making him swear to serve Arthur and also go to Camelot and apologise to Lancelot.

Gareth later counsels Lyonesse to report to King Arthur and pretend she does not know where he is; instead, he tells her to announce a tournament of his knights against the Round Table.

Eventually, Lancelot's unintended and brutal killing of his young friend and hero worship follower makes the central event of the final grand tragedy at the end of Malory's tale.

The legend of Gareth and Lynette has been reinterpreted by many writers and poets, the most renowned being Alfred Lord Tennyson in Idylls of the King (1859–1888).

Theodore Goodridge Roberts authored the short story "For to Achieve Your Adventure", in which Lynette knows she is sending Gareth into an ambush in an attempt to make him give up for his own protection.

In the Fate franchise, Gareth is depicted as a female knight, while keeping much of the legend's original family traits and story; she ends up brutally killed by the mad Lancelot.

An alternative Gareth also appears in a faery world in the "Lostbelt 6: Avalon le Fae" scenario of the mobile game Fate/Grand Order.