While the opportunity to heal the "sick king" lies with Perceval in De Troyes' account, Gawain himself is skilled with herbs that heal:

[H]e turned back [...] towards the oak tree where he had left the maiden and the knight who was in great need of a doctor to heal his wounds. And my lord Gawain knew more about healing wounds than anyone. In a hedgerow he saw a herb that was excellent for relieving the pain of wounds, and he went to pick it. After picking the herb, he rode back [...] My lord Gawain dismounted and found that his pulse was steady and that his mouth and cheeks were still warm.

'Good maiden,' he said, 'this knight is alive, you can be certain of it,for he has a steady pulse and is breathing well. And if his wounds are not fatal, I've brought him a herb which, I believe, will be of much help to him and will relieve some of the pain from his injuries as soon as he feels it. One cannot place a better herb upon a wound, for according to the book its strength is such that if it is placed on the bark of a tree that's been damaged, as long as it has not withered completely, the roots will grow again the and tree will once more be able to leaf out and flower.' (465-466)

Selections from Chrétien De Troyes' Arthurian Romances. Translated by Carleton W. Carroll.