You're on the right track with guy being more more gendered than guys, but I wouldn't go so far as to say guys is completely neutral. Take "Guys and Dolls" for example. A very dated example, but that specifically-gendered usage does still exist to a reasonable extent, like saying "it's guys' night" and so forth.
Basically, guys is neutral in those cases where it's used to describe a group that isn't entirely male but otherwise it's male (somewhat circular reasoning, but I can't think of a better way to describe it); guy doesn't usually get that same logic applied.
I haven’t heard the word gal used in person in a long time. “Guy” I hear frequently.
“Gal” seems derisive. I would assume someone was being berated or otherwise dismissed if being called “gal”. As in, “oh I suppose there are women here that I will address differently”, hence “gal”.
Interesting, I wouldn't place gal as derisive, just a word that has fallen out of common use. I used to work as a server and we'd talk about stuff like this.
Wonder if there is a regional aspect to it, or perhaps it being an older and less used word has lead to more individual variance in connotations?
