This answer is really a combination of two answers because one makes the other happen. When Gawain accepts the lady's gift and doesn't disclose it to the Green Knight, he has broken the chivalric code, a big no no for knights. That is why the answer here is violating the chivalric code. The acceptance of the gift is what brings on his breaking of the code.
Gawain can't really feel guilt over disappointing King Arthur because he has done what he told the King he would do: take a year and a day to find the Green Knight and challenge him to another battle. The same goes for accepting the Knight's challenge. If he didn't accept it, then he would probably feel guilt, but he is honor bound to accept the challenge, thus he can't feel guilt over accepting it.
