Character Analysis
An Old Boar
Gorlois is the first friendly face, outside of Cadal's, that Merlin sees when he flees the scene of his triumph at Vortigern's tower. He shows himself to be a jokester, though an old friend of Ambrosius:
"You have the look of [your father], Merlin Ambrosius." I heard him laugh again, deep in his throat. "And by God, there are times when you have the look of your devil-sire as well!" (III.12.57)
Yep, he's a jolly old soul. And he's ready to help Ambrosius to take up the crown of Britain. He appears again at Kaerconan, where Hengist the Saxon is taken. Merlin watches it all from a hilltop, impressed by the old man's presence on the battlefield:
The leader of the left wing I did not immediately recognize; a grey horse, a big, heavy-built figure striding it, a standard bearing some device in white which I could not at first distinguish. Then I saw what it was. A boar. The Boar of Cornwall. Ambrosius' commander of the left was none other than the greybeard Gorlois, lord of Tintagel. (IV.4.9.351)
There's a play on words in this passage. Merlin sees the device on Gorlois' banner: a boar. It will be Merlin's destiny to bring the "Bear of Cornwall" into the world, a child who will be the great King Arthur ("Artos" = "Bear").
It's a mark of greatness that will miss old Gorlois, though. Because even though his wife Ygraine is to be Arthur's mother, Gorlois is not destined to be the father.
A Royal Pain
Gorlois' star should rise with the coming of Ambrosius to Britain. And, well, it does—for, like, a second. After Ambrosius dies, Gorlois really saves the brash King Uther's butt by giving him good battle advice. Without it, Uther would have been toast.
But loyalty is nothing in the face of Uther's sex drive. It doesn't take long for the new and handsome king to take a fancy for Gorlois' gorgeous young wife, Ygraine. Gorlois shows his superior sense of honor when he asks for Merlin's help in this sticky situation: "[…] if you could get him to let us go back to Cornwall without waiting for the end of the [coronation] feast. […] Would you do that for me?" (V.4.106).
By getting out of town before Uther can lay his hands on Ygraine, Gorlois will spare his family from dishonor and not have to kill the new king. Very handsome of him.
He can't possibly know that the star of another, bigger king is about to rise—and neither he nor Uther will matter much after that. The good old boar also can't know that Merlin, his friend in need, will have to betray him to make way for King Arthur.
As Merlin says after Gorlois is dead: they're all nothing more than the tools of "the god."
Gorlois of Cornwall's Timeline