The poem Sir Gawain and the Green Knight is a medieval text. Medieval texts, like the preceding period's (Anglo-Saxon literature), begins "in media res."
In media res is Latin for "in the middle of things." A characteristic of both Anglo-Saxon literature and Medieval literature is that it begins in the middle of things. The tumultuous opening simply foreshadows the events which are to come. For some, this may make the action of the text more light-hearted or not as serious as the action with which the text opens.This fact can be compounded by the fact that the poem moves into Christmas celebrations at the castle of King Arthur.
The conflict which Gawain faces in the poem is dwarfed by the action of the Trojan war. Some may find that this allows a reader to be a little more at ease with the lessons being taught in the text instead of focusing upon the intense battles of war.