Something to mention here is the involvement in what we deem as the "Beheading Game" which was actually a very common game in Celtic lore. However, in the story of Gawain and the Green Knight, there are other parallels to similar stories in Celtic lore. One of these being the Champions Bargain, Alice Buchanan explains the similarities, saying "1. As in GGK, the challenger offers to submit to a stroke with the ax on
condition that he be allowed later to return the blow. In the Champion's
Bargain, on the contrary, the exchange of blows is first proposed in the
reverse order, and only changed after Dubthach's protest.
2. As in GGK, the completion of the bargain is carried out in a wild
region, in GGK beside a turbulent burn, in the Yellow and Terror tale be-
side a loch. By contrast, in the Champion's Bargain the adventure con-
cludes in the royal palace.
3. As in GGK, the hero is led to the spot by a guide furnished by his
host. In the Champion's Bargain, of course, no guidance is necessary.
4. As in GGK, the challenger "let the ax come down three times on his
neck." In the Champion's Bargain he deals but one stroke." (Buchanan 317).
She goes further into the context of mixing lore and the similarities between Gawain, Lancelot, and other figures from stories that predate the Green Knight.
It is also worth mentioning how Green impacts the story. Symbolism is prevalent in the story, Green being used as a symbol of nature, the unknown, death, and even the Devil. As the Devil was depicted as a Green being.
Sir Gawain and the Green Knight is one of the few pieces of literature we have from the 14th century, J.R.R. Tolkien even translated the document. This story has given us a great cultural impact and insight into our past. Its even more relevant to add how previous stories and even pagan lore influenced a Christian story.
Buchanan, Alice. “XXI.” The Irish Framework of Gawain and the Green Knight, 2nd ed., vol. 47, Modern Language Association, 1932, pp. 315–338.