“Sir Gawain and the Green Knight” is a medieval romance (it inherits a body of Arthurian legends that had circulated in England for a couple of centuries) but also an outlandish ghost story, a gripping morality tale and a weird thriller. It is a sexual teaser that keeps you on the edge of your seat. It’s easy to imagine huddling around the fire to listen to it. You can tear through it in a night or two — I couldn’t put down Simon Armitage’s compulsively readable new verse translation — and linger over it for years.