Character Analysis
Gareth is the second-youngest brother, but his bro Gawain refers to him (when he's practically a middle-aged man) as "our wee little brother" (C.8.304). Not a lot of respect going on here.
As a child, Gareth is just as clannish and insular as his siblings. But this dude is kind of different from his more bloodthirsty brothers. Overall, he's generous and has a strong sense of justice (Q.1.57). He's also tender-hearted. During the unicorn incident, Gareth "began to cry in earnest—he did not know why" (Q.7.194). Why? Probably because your brothers are butchering a magical being, guy. You have every right to cry about it.
Wants to Make It on His Own
Gareth comes to Arthur's court much later than the rest of his clan. He hides who his family is, and doesn't tell people his name (so, he's the Emilio Estevez of his family, wanting to make it on his own without the illustrious name). Since he doesn't admit to nobility Sir Kay sticks him in the kitchens, and sticks him with what is meant to be an insulting nickname: Beaumains (Pretty Hands).
Luckily, Lancelot sees Gareth and just knows he's knight material. So he oversees his knightly education, and even knights him personally (which Gareth considers a huge honor). He regards Lancelot as a sort of father figure. Unfortunately, Lance ends up accidentally killing him and Gareth (oops), which sets off the entire feud between the two families that leads to the final destruction of the Round Table.