Is "guy" gender-neutral?

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?

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."