I think we should examine what Sir Gawain represents first. Gawain is the perfect medieval knight. He is Arthur’s loyal servant and we see this in the poem. It was against the knight code of chivalry for Arthur to accept a challenge from anyone while his men sat back chowing down on chicken or pheasant. Actually it took Gawain awhile to step up to the plate. It was only after the Green Knight mocked Arthur for a while, basically calling him girly pants, that Gawain took the Green Knight's challenge. Still, as this was part of the morality/ honor code of Round Table Knights, Gawain does his moral duty.
Chivalry, and the morality involved, is tested further when Gawain gets to the Green Knight's castle. The deal is, of course, to exchange anything each man obtains while the master (Green Knight) goes hunting. Mrs. Green Knight (sorry I forget her name) attempts to seduce Gawain. Now it's not that Sir Gawain didn't consider jumping into the sheets with the Knight's wife, apparently she was not bad looking. Gawain was constrained by his damn moral code of chivalry, so he settled for some kisses instead (good thing he was not Tiger Woods!) The Green Knight is really a Pagan fertility/ vegetation God who comes to test the Christian morals of these super knights .These morals are what save Gawain's life in the end. Apparently The Green Knight doesn't mind Gawain borrowing his wife's clothing (the invincible garter belt) and is impressed with Gawain’s honesty. So, if you’re a knight, remember this little morality tale in case this happens to you!