I'm finally getting around to watching Green Knight. The movie's about the triangulation of desire. Very straightforward Girard stuff.

The King recognizes his lack of peers (either among his Saxon rivals or his court) & desires the company of his sister as a neer-peer: he triangulates this through Gawain. G "externally" mediates the King's desire. G himself wants to be one of the knights, to have his desire "internally" mediated through them.
...

So the Green Knight appears in the context of this as this figure of reciprocation and equivalence. Knick for knick, blow for blow, year after year, and so on.

Later on "The Lady" will pose an interpretation of greenness that is given without any irony from the film, so I think it's basically an endorsement of the filmmakers' perspective no the greenness of the knight.

Green, in the film, is associated with perspectives that are distanced from the mediation of desire. ...

The examples of greenness given in the monologue are birth, infection, and corruption: birth following from desire; infection and corruption indicating the death of desire.

The thing is, though, I think it's better to understand Green in terms of the woolly pagan perennialism that was being driven out of Britain by Christendom: the Priest could tell you about the Trinitarian mediation of desire; the druids could tell you about the smooth cycles of geronition & death.

Another word on the setting: you've got death as a phase of geronition (greenness) contrasted against the Christmas story, which is the commemoration of the paradoxical birth of the paschal sacrifice who will transcend death through death, the Christ.

The witchcraft of the green garter suggests a third way out: life sustained by human will until it becomes unbearable (such as through watching the deaths of others), until life must be renounced.