It's a cultural thing and varies hugely depending on which country you're in. In my bit of Britain it's quite common for groups of women to refer to themselves as 'guys'.

Guy Fawkes was male, and there is a feminine equivalent gals, but I think guys is slowing moving over into gender neutral territory (it wasn't when I was a kid, but times change).

In the US, it depends heavily on context. "The guys" or "those guys" almost always implies males. Same with any singular usage of the word "guy".

"You guys" is the only usage that can refer to a mixed-gender group. Obviously, that isn't how GP used it.