Maybe it's a regional thing, but I certainly don't consider "guys" to be gender specific. If I were talking about a group of exclusively males, I would say "boys" or "men".
Or, in some regions and in colloquial conversation with a group of male friends, "dudes".
I heard three young women refer to each other as "dude" in the Portland airport a week ago. It seems to be a generational thing.
It's interesting, I don't consider it gender specific either, but it does sound more fitting when directed at an all male group or to single out the males in a mixed-gender group.