Sir Gawain has to face two challenges. Why did the narrator choose those two challenges specifically and in what manner do these challenges oppose/relate (to) each other?
Sir Gawain cannot become a true knight through bravery alone - there is more to it than sheer brute force and ignoring danger. To be a knight, a young man must also be, or become, a gentleman first and that process is the main thrust of this work. Many wannabe knights would have had a little education in a rich man's house and some may have thought their apprenticeship was done. However, the favours of young ladies and the award of knightly status by genteel patrons was not so easily achieved. The education in culture, chivalry and 'noblesse oblige' first had to be tested, and there were many temptations along the way. One reason for the narrator's challenges is the idea of resisting temptation. Another is regard for the law.
Gawain agrees to the Green Knight’s proposal to behead the Green Knight but he survives the beheading. Before it is his turn have an appointment with the knight for his own beheading, Gawain embarks upon a journey through the wilds to find the Green Chapel. He discovers a castle where he will encounter many temptations before he can keep his grisly test date. One problem is that his host has a wife whom others consider attractive, so Gawain’s manners, chastity, courtesy and honesty are tested. After battling with his inner self, he finally meets up with the Green Knight on his arrival at the Green Chapel. The Green Knight does a couple of mock axe swings first, then manages to hit Gawain the third time, but it turns out that Gawain has cheated death because the weapon has only scratched his neck. In a way, in this test the narrator shows that the second test (involving promises and laws) is rewarding Gawain because he was at least able to fulfil his part of the bargain by turning up for his 'just desserts' or to face the music/consequences.