Posting an answer in 2020 since the question — and accepted answer — date back to 2011 and I believe some things might have changed between now and then.
While “gals” is technically be the feminine version of “guys”, I would recommend avoiding it if at all possible.
This is especially true in business and professional settings where people are not necessarily there to be “friends” but you aren’t enemies and you ultimately have to work together in some functional way.
While Caleb’s answer — the currently accepted answer — is well thought out and provides some decent rationale, I find it a bit problematic. At its core, the word “guy” is clearly defined as “A man.” in the Oxford English Dictionary and “Man, Fellow” the Merriam-Webster dictionary, the Merriam-Webster definition also accounts for a plural “guys” by stating the following:
“Used in plural to refer to the members of a group regardless of sex.”
The fact that the singular is gendered and the plural is neutral might not seem like a big deal, but in many ways it makes no sense as a general rule and — honestly — seems confusing.
So I did some research and found some pertinent online posts, articles and resources that discuss the issue. My personal big takeaway from this stuff is that in casual use, “guys” is an acceptable gender neutral plural. But in more formal business and professional settings but should be avoided.
For example, this Vox piece from June 2015 quotes Jeane Anastas, a professor of social work at the NYU Silver School of Social Work who states:
“Whatever Webster’s dictionary says about the plural ‘guys’ and despite the fact that I sometimes catch myself saying ‘you guys’ to people of all genders, ‘guy’ is a gendered word.”
And this August 2018 article in The Atlantic refers to this 2002 essay by Sherryl Kleinman, a former professor of sociology at the University of North Carolina at Chapel Hill, titled “Why Sexist Language Matters (PDF)” which states in simplest of terms:
“That women primarily exist in language as ‘girls’ (children), ‘sluts’ (sex objects) and ‘guys’ (a subset of men) makes it less of a surprise that we still have a long list of gendered inequalities to fix.”
This piece in ABC Life from August 2019 focuses on the business and professional side of things with this quote from Lisa Annese from the Diversity Council Australia:
“In the workplace, you cannot reasonably predict the impact that your words have on other people. If you’re a leader and you’re addressing a whole group of people, isn’t it better to use a more accurate term?”
And this April 2019 post from HotJar’s blog echoes the sentiments of how even if the intentions were good; the effect can still be negative to those who don’t identify as “guys”:
“Even though most people who use the term don’t do so with the intent of it being sexist or exclusive of women, it can and often does cause women to feel left out of the conversation.”
The overall point being that while “guys” is currently considered gender neutral to some, the fact that it’s singular is clearly gendered makes the word truly problematic in the literal sense.
So my advice is to just use gender neutral — yet welcoming and friendly — phrases like these when dealing with business and professional situations:
- “Hey everybody!”
- “Hey people!”
- “Hey all!”
- “Hey y’all!”
- “Howdy folks!”
- “Welcome team!”
- “Ladies and gentlemen!”
And in casual situations? That is your judgement of course. Heck, you can say something like, this in in casual situations:
- “Hey jerks!”
- “Deadbeats! How are you doing?”
- “How do you do, fellow kids!”
Because at the end of the day, being a jerk, deadbeat or an old creep pretending to be a kid is a genderless role.