I think it seems like Gwen should be short for something, but since it has a meaning on its own it could definitely work as a full name based on my own definitions. :)
I'm really torn on what I think of Gwen. On one hand I think it's elegant but quirky and upbeat, but on the other hand I strongly associate it with one of the most obnoxious people I've ever met (this person's wife was named Gwen, and her behavior played into his obnoxiousness).