If only there ever is a need to describe Sir Gawain with one word, this word would be nobility. All his actions are noble. His only bad deed is lying to Sir Bercilak in the Green Castle when Gawain did not return a girdle he received from Bercilak's wife on the third day of his visit. However, this situation practically forms and defines Gawain. It is important to remember that fight with the Green Knight was Gawain's first challenge; he was the youngest knight of King Arthur's court, a knight with no experience behind him. Gawain suffers for lying to the Green Knight (the third blow of an axe cuts Gawain's neck), and this experience influenced Gawain so much that he keeps and wears the belt as a reminder of his mistake even though everybody at the Arthur's court take this as a fashion statement when Gawain returns.
Gawain looks and speaks in the way an ideal knight should look and speak. His clothes are regular for the knight; his speech, on the other hand, is somewhat distinct from other knights. He is the only knight that steps forward to save Arthur's honor and life in the stories of The Wedding of Sir Gawain and Dame Ragnell and Sir Gawain and the Green Knight. In the story of Dame Ragnell, Sir Gawain does not hesitate to agree to marry Dame Ragnell, the foulest woman alive, to save King Arthur even though Arthur does not ask Gawain explicitly. Gawain feels honored that he is able to help his lord in the moment of trouble. In the second story, even though Gawain is the youngest knight he steps out when all other knights are frozen and Arthur is embarassed before the Green Knight. Both these episodes show Gawain's true devotion to his Lord and code of knighthood.
In the early stories, Sir Gawain is often considered to be the noblest knight of all. In later stories, like Morte D'Arthur by Sir Mallory for instance, he often loses the first place of Sir Lancelot. However, Sir Gawain can still be considered the noblest knight since Sir Lancelot after all does have an affair with Arthur's wife, and even though chivalrous code does not tie love and marriage together, it still does not look good on his resume. Sir Gawain, on the other hand, is always follows the code, and tries hard to serve his lord to the best of his abilities. The idea of commitatus, an agreement between a warrior and a lord, is very strong with him.
Despite his little flaws, I consider Sir Gawain to be the perfect knight, an example on how men are supposed to behave. Even though knights are long gone, it is not a reason to stop following the code of honorable conduct, following the steps of Sir Gawain.