Firstly, he is the nephew of King Arthur. His mother was Arthur's half-sister, Morgause, and his father was King Lot. Lot was the king of Orkney and Lothian. The family boasted of five boys altogether -- Gawain, Agravain, Gaheris, Gareth and there was a half-brother called Mordred...you may have heard of him??!
In Prose Lancelot he is described as;
Shy - not one for making speeches,
Had a temper (although not as bad as Gawain's),
His right arm was longer than the left.
He isn't portrayed in the best of lights in the work of Malory's Le Morte d'Arthur -- there is that terrible incident where he was involved in the revenge killing of King Pellinore (Pellinore murdered Gaheris's father). He beheads his own mother when he discovers her in a very comprimising postion with Pellinore's son, Sir Lamorak. To make matters worse, Gaheris blames his mother murder on Lamorak who is then hunted down and killed, by Gaheris's brothers.
This 'act of revenge' goes against the knightly code and when it is discovered that Gaheris murdered his mother, he is banished from court.
For some reason he reappears later on in Malory's story. Guinevere is to be executed for her affair with Lancelot. Arthur expects that Lancelot will try and rescue her. He asks Gaheris, Garath and Gawain to protect the pyre. Guinevere must died. Gaheris and Garath reluctantly agree to, but Gawain refuses.
As expected Lancelot attempts a rescue and is sucessful, but because Gaheris and Garath are not wearing their armour, Lancelot does not recognise them and cuts them down.
When Gawain learns of his brothers death, his grief and rage are great.
The death of Gaheris and his brother marks the begging of the end of Cameot and her knights and the rest, as they say, is history or legend, or whatever else you want to call it...!