Gareth arrives in Arthur's court incognito and makes three requests of Arthur; one, to be allowed food and lodging in the court for a year, and the other two to be granted at a later date.
Kay gives Gareth the nickname "Beaumains," or "pretty-hands," because his hands look like they have never done a day's work.
About a year later, Gareth asks to be allowed to help a lady who seeks protection for her sister from a knight who has besieged her (that's request number two) and to be knighted by Sir Launcelot (request number three).
Gareth proves himself by defeating a family of four knights, one of whom is the knight besieging the lady's castle. Mission accomplished.
He marries the lady of the Castle, Lyonesse, and becomes a Knight of the Round Table.
Refusing to take part in his brothers' feuding, Gareth chooses instead to align himself with Launcelot and his followers rather than the feuding faction of his family.
Arthur forces Gareth to be part of the force that takes Gwenyvere to be burnt at the stake. He refuses to be armed because he doesn't really want to take part. Knights never hurt ladies, after all.
Unable to recognize him because he's not wearing his shield, Launcelot kills Gareth by mistake.