The comparisons between TGK and Excalibur are due to environmental themes, but they're still different; Excalibur is about how Arthur and the sword are "tied to the land," represented by that "coils of the dragon" thingy Merlin takes Helen Mirren into. I watched that film (and initially hated it) for a Fiction into Film class I took in college and when I asked what the film had to do with what was going on in the world in 1981 the professor answered "environmental movements." TGK's environmental themes are more about the hero, Gawain, getting swallowed up by the natural environment, represented by the Green Knight, who looks like a tree, as opposed to doing something heroic, which is the big theme of the movie. Honestly, he's treated worse than most Coen Brothers protagonists. Or antagonists, for that matter. This is what's put me off so far.