Take a story's temperature by studying its tone. Is it hopeful? Cynical? Snarky? Playful?
Sir Gawain and the Green Knight is a fairy-tale-like story, full of magical creatures and supernatural happenings. Accordingly, the tone of the story is somewhat fairy-tale-like, or fantastical, as well. This kind of tone involves a lot of hyperbole: exaggerated descriptions of people or things as the biggest, best, or fairest of them all. King Arthur’s knights are the "most famous warriors in Christendom," his ladies "the loveliest who ever drew breath," he "the finest king who rules the court" (51-53). The Green Knight is "the largest" of all men "and the most attractive of his size who could sit on a horse" (141-142).
For the most part, the narrator seems caught up in all the awe and amazement at this exceptional world and the marvelous events that occur in it. He is full of praise for Gawain’s exceptional skill and virtue, the beauty of Lord Bertilak’s palace and lady, and of Lord Bertilak himself. More importantly, he remains silent at crucial narrative turning points – for example, when Gawain hides the green girdle from Lord Bertilak. This silence means that the narrator avoids ever having to criticize his hero.