What is Gawain’s ultimate takeaway from his experience? Do his brother knights agree? What does this say about knightly behavior?

I think that the ultimate value of Chivalry is important here. Perhaps the most significant of these human constructions is chivalric code which forms such an essential part of medieval literature and of Gawain's belief system. Gawain is the very embodiment of chivalric values, yet his encounter with the seductive Lady Bertilak forces a crisis in the chivalric value system: should he honor the requests of the noble lady or remain faithful to his lord? Upon his return to Camelot, King Arthur does not even detect the moral crisis within Gawain. And most unexpectedly, the "test" of Gawain's chivalric values have been in fact a game engineered by Morgan le Fay for a less-than-noble purpose. Disillusioned, the once-idealistic Gawain finds that the code of chivalry which once formed his moral core has now been shaken.