I finally watched this and aside from the Indians at King Arthur's Court, which was very distracting, it was OK.
Seeing Indian people as members of the royal family in Arthurian Britain was just like watching John Wayne playing Genghis Khan in The Conqueror: Utterly ridiculous.
As for the movie as a whole, I can only conclude that the people responsible made this as some sort of John Boorman tribute, because it looks and feels like a psychedelic stew of Boorman's Zardoz and Excalibur, only somewhat less involving than either. In any case, those two films are the only other movies I can think of that are remotely similar to The Green Knight.