Not sure how much help I can be...but I seem to remember that Sir Gawain etc. Is mentioned in Fraser's Golden Bough as an example of the continuation of Jack-in-the-Green type anima. The green knight is an example of a tree spirit. Or so he thinks.
Im always shocked by the girdle sequence, though. Gawain borrows a magic girdle from the green knights lady (sister?) and then "wraps it around himself three times" How big was this woman?
Sources to look up may include Tolkein, who did an analysis of it.