One guy is always male. (Which isn't surprising, since Guy has been a man's name for over a thousand years.) Therefore it follows that if someone says "It's a guy thing" or "Guys' night out" you know with 100% certainty that said guys are male.
But I have seen a woman address a group made up entirely of women with "Hi guys!" in which case those guys are now 100% female. However, even though a group of women can be called "guys", I have never seen that group subdivided such that one women would be a "guy", regardless of how logical that might be.
Interestingly enough, I have witnessed both genders refer to a diverse collection of both men and women as "guys", and neither male nor female has been offended by this.
So not only is it one of the very few words in English which is gendered in the singular but genderless in the plural -- the only other of which I can think of is "man/men/mankind" -- it actually tops it. Many women feel that the word "mankind" is sexist when used to refer to all humanity, but I have yet to see any woman seriously object to "guys" even when used in nearly the same way.
I don't really have a point to this other than Huh. A distinctly gendered noun has become a gender-neutral collective through cultural drift. Who'da thunk it?
PS: If the title of this blog post immediately made you think of this, then congrats! You're old, like me.