I don't think the Green Knight in Sir Gawain and the Green Knight is a figure of evil at all. He entered King Arthur's court uninvited, it's true, but he came peacefully. He issued a challenge, and once he had a "taker" he took the first blow--a blow so strong it separated his head from his shoulders, in fact. He was a gracious host to Gawain a year later (though we didn't know it was him, at first). He kept his word and actually showed grace to Gawain; he could have given him the same stroke of the blade which he had suffered in King Aurthur's court. After the entire incident was over, he invited Gawain back the next year, a sign of respect and gracious hospitality.
What he probably does symbolize is a test of honor: who would rise to the challenge, would he keep his word even in difficult circumstances (remember Gawain had to find him without knowing where he lived), how would Gawain deal with temptation. Gawain passed the test; and, since he's a member of King Arthur's court (and is actually a cousin to the king), he brought honor to the entire kingdom. I just don't see any way the Green Knight represents evil in this story.