Guy has followed an improbable path from its origin as an eponym for Guy Fawkes to its common and versatile use today. It’s increasingly popular as a term to address mixed-gender and all-female groups, but not everyone welcomes this development (see video below). So how gender-neutral is guys, you guys?
Instead of a simple answer there’s a spectrum that depends heavily on context. But we can draw some general conclusions, as I did in an article at Slate’s Lexicon Valley on guy(s) as a gender-neutral word: