Is it used for both sexes in vocatives only? I still see people using it to mean 'a man', as in 'I was friends with mostly guys', while I thought it had come to become gender-neutral. Â
Its meaning is very sensitive to context at this point. Historically, it would refer pretty strictly to males. Now, there are some clear cases in which native speakers use it as a vocative to refer to either a mixed group or even a group of all women. Outside of the vocative, the examples are few and tend to be controversial. Many think such uses negate the women in the group.
Much like the vocative, many uses of "you guys" could easily be mixed or female. Ex. "Did you guys hear that Chris and Pat had a baby girl? Didn't Quinn let you guys know?"
But it is still useful and used to refer specifically to males, sometimes in pointed distinction from the females. Ex. "We all went to my sister-in-law's house, but the guys just watched the game in the back room."
To non-native speakers, my advice to avoid trouble would be to use it only for males, but understand that it could refer to men and women, especially in the vocative.