Quote 1
"I accept it gratefully, not for its wonderful gold,
Nor for the girdle itself nor its silk, nor its long pendants,
Nor its value nor the honour it confers, nor its fine workmanship,
But I shall look at it often as a sign of my failing,
And when I ride in triumph, recall with remorse
The corruptions and frailty of the perverse flesh."
(2430 - 2435)
Gawain accepts the girdle as a sign of the frailty of the "flesh," or body. He failed to disclose his receipt of the girdle to Bertilak because he was too attached to his own life. He gave in to an animal instinct for survival rather than following the rules of civilized society. Accordingly, instead of seeing the human ingenuity of the girdle, its monetary value, or the status it might confer in society, Gawain links it to the sins of the flesh.
Quote 2
"I am the weakest [of your knight], I know, and the dullest-minded,
So my death would be the least loss, if truth should be told;
Only because you are my uncle am I to be praised,
No virtue I know in myself but your blood."
(354 - 357)
The idea that a person’s virtue might reside in their blood was a medieval one that justified systems of familial succession. It’s unclear if the modesty Gawain displays throughout the poem - calling himself the "least" of Arthur’s knights when he’s known as the best - is a false one or just another part of his virtue.
Quote 3
"I am at your commandment to act on your bidding,
As I am duty bound to in everything, large or small,
by right."
(1039 - 1041)
Gawain puts himself at the disposal of his host in everything, something which both of his codes of conduct - courtoisie, and chivalry - require him to do. Since he has accepted the hospitality of Lord Bertilak, in courtesy he owes him a debt of gratitude as a guest. And since Lord Bertilak outranks Gawain, he becomes a substitute liege lord of sorts in Arthur’s absence, to whom he owes the same obedience and loyalty.