The plural "guys" definitely is, at least here in San Francisco — I'm often hearing all-girl companies here being greeted with 'Hi guys, how are you doing?'.
How about the singular guy? Is it universally assumed that 'the guy who will be doing this' can be either guy or gal?
"You guys" is a familiar, all-inclusive way of addressing a group of men or women directly. That said, there are some important distinctions you must understand.
"You guys" is more likely to be said in women => women or men => men or women => men or mixed-group => mixed-group contexts. It is less likely to be used in men => women contexts, but is still heard and would probably not occasion any confusion or merriment.
The singular "guy" is another animal. It refers to males. It is also used to draw gender distinctions in a general way.
A guy walked into my store and asked for some cigarettes.
There is no doubt that this is a man we're talking about.
In most plural usages that are not directly addressing a group, this rule also applies.
Guys are pretty simple, when you get right down to it.
This will also be understood to refer to men only. If you wanted to make the same statement about women, you would use another noun: women, gals, whatever.
However, you could say something like
My friends in San Francisco? Those guys are so crazy!
Now we're not sure we're talking about men. If the speaker is female, it might mean a group of women. Note that I say might. It is more likely she would still be talking about men or a mixed group, but you never know.
It's a hard word to pin down. Much depends on context.
Addendum
On a walk yesterday evening I encountered a woman walking three dogs. When we got close the dogs started barking at me. The woman and I exchanged greetings, and then she admonished her dogs by saying: "Cut it out, you guys!" One dog kept barking, and she said: "Sally, stop it!" What to make of this? I suppose that for some people "guys" can refer to groups of dogs as well as humans, and not just male ones. Ain't English fun?
New Evidence (2023)
A young woman scientist who does YouTube videos just referred to herself as "a dark-matter guy" in a video that criticizes string theory. This is another reinforcement of a trend that shows that, 12 years after I first wrote this answer, even beyond the usages I described, "guy" is taking on a genderless sense quite independent of its original use as a gendered expression. Language is always in motion.
John McWhorter says:
(John McWhorter is a famous linguist who has written many books and now writes for The New York Times on matters of language)
“Guys” has undergone a similar transformation. The word has come to mark “you” as plural (note that we don’t say “you guy”), a very handy grammatical function that many other languages address with a single pronoun. “Guys” can still mean “a collection of men,” but it also means something quite different. . . .
But why “guys”? We wouldn’t address a room full of men as “you girls.” One reason is that English lacks a warm, relatable word for women. “Ladies” is dainty. “Sisters,” beyond Black English, sounds like nuns. “Gals” is a touch comic and perhaps a tad vintage — a little hair spray and Jell-O molds, to my ear.
When women call one another “you guys,” “dude” or “bruh,” I don’t hear a fear of being women. They have sapped the gender from a masculine word in order to use it for their own purposes. I hear it as progress.
'Nuff said, you guys.
TL;DR - 'guy' is not gender neutral. 'You guys' is accepted in colloquial speech to fill in the lack of a common subject in the second person plural. It is not acceptable to use that phrase in writing or formal speech.
The word 'guy' is not gender neutral, let's start there. It very much only refers to the male gender. But there is a very important caveat to that statement in regards to its usage in the second person plural. English doesn't have a good form of the second person plural. The closest thing we have is just 'you all' which of course in the south (US) is used very often as just y'all. In places where y'all is not said, saying 'you guys', can be commonly heard when informally addressing a large group, regardless of gender. I notice that I use it often when addressing my co-ed soccer team (made up of men and women) 'you guys are doing great out there!'. Still, this is not acceptable in writing or in formal speech.
I would say that "guy" is somewhat gender neutral, at least in "gender neutral" contexts. Example: "Those guys are getting something to eat." Those PEOPLE are getting something to eat. (Everyone needs to eat.)
But, "Those women are all dating GUYs." The context is not "gender neutral." It's clear that those women are all "hetero."
Although guys is usually gender-neutral, it's not always so. The title of the Frank Loesser musical
Guys and Dolls
is enough to tell you that it can sometimes be used to refer just to males.
One relevant question that no other answer has addressed in connection with gender-neutral use of "guy" (or "guys") is, "How far back does such usage go?"
I just watched a movie called Three Broadway Girls (aka The Greeks Had a Word for Them), which was released in 1932; and on several occasions in the movie, in scenes where only the three lead characters are present, Jean (Ina Claire's character) refers to Schatzi (Joan Blondell's character) and Polaire (Madge Evans's character) as "you two guys." You can see and hear one instance at 7:09 of this YouTube video of the movie ("Is there any wonder I want to see you two guys?"), another at 8:08 ("You know, I don't believe any girl ever had any better friends than you two guys."), a third at 54:14 ("I always liked you two guys."), a fourth at 1:08:32 ("Hey, cut out the personal remarks and tell me what you two guys are doing here."), and a fifth at 1:11:41 ("Not as much as I like palling around with you two guys.").
The movie script was based on Zoe Akins's Broadway play The Greeks Had a Word for It (1930). I haven't been able to find a copy of the play's script, however. In any event, if the movie reflects contemporaneous usage, it appears that some women were referring to other women as "guys" at least 86 years ago.
I don't think there's a definitive answer because the word is evolving. I base this on the above, and on a comment from a late-20s female acquaintance (Connecticut, professional): admitting she's often unimpressed with well-received movies, she pointed to herself and said "I'm that guy." I realize this is a current idiom, but I was surprised that she showed absolutely no self-consciousness or irony about the statement. It's the first time I remember hearing a woman refer to herself as "guy." (Granted it was a casual comment, but when people are being formal do they use "guy" at all?)
Now that I think about it, this may be a strategy (conscious or not) by younger women to get around any sexism of "guys" -- if women are guys, the sexism disappears.
I agree with the posting by Innate that guys is most definitely not gender neutral, since we have gender neutral words in English that are much more appropriate such as you all, you folks, everyone, you people etc. The word guy is singularity male and the word guys can only be assigned gender neutrality if it wasn't used to describe men specifically, and which 99 percent of articles that refer to men uses the word guys. If the word men is considered sexist and non-inclusive, the word guys is right beside it.
I am female and speak British English. I would not use "guys" to address a group that contains women, and in fact would find it somewhat sexist. However, please note that I am 70 years old, and it is quite possible that younger people might use it in the way described.
