How does the court react to Sir Gawain's decision to leave?
The challenge that Gawain accepted from the Green Knight was simple: the Green Knight would submit to a blow from Gawain, if Gawain agrees to submit to a similar blow in a year and a day. Gawain's blow decapitates the Green Knight but incredibly does not kill him; the headless body picks up its head, reminds Gawain of his oath, and rides out of the hall.
When the time comes for Gawain to seek out the Green Knight, he goes to Arthur and declares his intent to leave. The other knights gather round him to mourn his departure ("In dole so drear their tears in hall together blend/To think that good Gawain must on such an errand wend."). Gawain requests that he be armed, and the poem deals at great length with the details of his armor and its decoration (see Book II, sections 4-7).
After he is gone, the court is plunged into melancholy ("the hearts of all did sink") and begins to question the wisdom of Gawain's quest. Most think Arthur is wrong to let Gawain leave, and it would have been better to trick the Green Knight or simply ignore the terms of the deal.
By Christ, 't were pity great/If yon good knight be lost, who is of fair estate;/...T' were better to have wrought by wile, methinks, than might!.../Did ever any king obey such a strange behest,/As risk a goodly knight upon a Christmas jest? [Book 2, Section 8]
The court weeps for Gawain, but he leaves nevertheless. However, the criticism of Arthur here suggests that Gawain's sense of duty and honor is foolhardy or misplaced, and this question about honor and judgement is one that will recur later in the poem.
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