"Gal" unfortunately has a jaunty 1950s sound to it that "guy" doesn't.
That's an issue, or sort of a western feel. My dad always used the term, and he was of that era. It does pop up out here in CA sometimes, though. Maybe it's the complete lack of informal, non-derogatory or pedestal-izing words for women that drives it? They have "lads" and "lasses" in parts of the UK, but that never caught on in the US.
There are times to use "woman" and the times to use "chick," and it's all dependent on voice and context.
"Chick" was always fairly derogatory in my part of the country--something men called women when they were objectifying (let's go out and pick up chicks), or something people would use when they wanted to imply that the woman or girl in question was rude or lacked sophistication (so this
chick comes up and gets in my face about...).
The strangest one were the people who pronounced the word woman "woah-man," though.
It would usually be in a sexist context, like a boss I had years ago who had a story about boarding a plane and realizing that there was a "woah-man" pilot, or saying something like, "There's this ugly, rude "woah-man" who works at the DMV." I haven't heard it for a long time, and it was mostly people in my parents' generation or older who did it. I think the implication was that she wasn't behaving in the expected feminine way (or appearance), hence the deliberate mispronunciation of the word, since she wasn't a proper woman?
For fiction writing, though, you really have to refer to men and women the way the characters in the book would. In contemporary college, rightly or wrongly, college students often refer to women undergrads as "girls," whereas they call man undergrads as "guys."
However the universities themselves have been using the terms "men" and women" in an official sense since I was an undergrad. "Men's residence halls" and "Women's residence halls," the "federation of university women," "Women's and men's basketball programs," "Women's health clinic," etc.