Who is Sir Gawain and the Green Knight author?

Sir Gawain and the Green Knight would usually be considered too short to qualify as an actual epic. The action lasts only one year (from one New Year's Eve to the next) and really covers only a few days out of even that short timeperiod. Sir Gawain also has a much smaller scale than most epics. An epic typically will have several dozen important characters; Sir Gawain really has only three main actors (though each of them is developed in close detail).

The sort of poem Sir Gawain is, is usually called a 'Romance': a story with a single well-defined plotline, and a very limited setting and set of characters.

Romances were very popular in the Fourteenth Century (when Gawain was written); many of Chaucer's Canterbury Tales are romances, and there are also others including Havelok and Undo your door.

To compare these two forms with something a bit more modern: an epic has a large scale, a big story, and many characters - like a novel; a romance has a smaller scale, few characters closely observed, and a straightforward story with a beginning, middle, and end - a romance is more like a short story.