It was not Gawaine’s discovery, of course; it was simply a plant he knew for its remarkable healing properties. I am not aware that herbal lore was included in the usual knightly education, though it probably should have been: fortuitously finding this herb near the Galloway border, Gawaine used it on the wounded Greoreas. Gawaine explained that it was potent enough to heal a sick tree (providing the tree had not entirely dried).
Considering how well and quickly it restored Greoreas, this might not have been an exaggeration.