A guy who looks good could be a compliment to someone who didn't dress well before and dresses better now. It could also refer to someone who looks good in what they're wearing or how they carry them self at the moment.
A good-looking guy is someone who is attractive, handsome, etc.
Imo I think when someone says you look good it means you look presentable and neat and it (that "it" could be clothes, a new haircut, shoes etc) just makes you look personable. When someone says you are good looking I think it means you are nice to look at and you are physically attractive.