Short version: I have a very particular set of skills. Skills I have acquired over a very long career. Skills that make it possible for me to enjoyably extract 95% of the meat of THE GREEN KNIGHT without difficulty. …Your mileage may vary.
Slightly longer version: have you read SIR GAWAIN AND THE GREEN KNIGHT? As in, voluntarily, and with enjoyment? Then you will have no trouble whatsoever with THE GREEN KNIGHT. It will be an open book to you, so to speak, and you will get to enjoy an excellent movie that is steeped in that particular myth, with no incomprehensible deviations from the text. You’ll enjoy it.
If you have not read the source material, though… yeah. I won’t blame anybody in the slightest if they came out of the movie theater going “Huh?” Even the ending ruthlessly assumes you’re familiar with the original work. Mind you, I’m fine with that, but how does A24 expect to make their money back on this one?
Bottom line: if you like Arthurania, go see this quick before it goes away.