Just to nitpick, guy and dude are not gender neutral, they just happened to be heavily used by the community in general, mostly because people assume users are men by default.

Speak for yourself. A lot of people use it as a real gender neutral.

There are two related but subtly different uses here. A word like 'he' is a default, which can lead to problematic implications. A word like 'guy', to some people, is a true neutral, implying gender no more than the word 'somebody'.

Even if we were talking about 'he', your link would be out of place. Calling out the use of a gendered default as bad is not in fact another point of view on whether it is the default.

I have never heard of a female being addressed as "guy". Phrases such as "hey guy", "what's up guy" and so on have always been directed to a male.

Sometimes I will use it in a conversation when referring to a third party unknown to the person I am talking to and the subject's gender has no bearing (like in my first comment).

For example, "Oh, I know a guy that knows X," "I know a guy that is a fan of X" or in a less positive manner "That guy just cut me off [on the freeway]!" (since you rarely see who it might be and their gender doesn't matter)

I could say "dude" as well, but sometimes dude might be too informal. I could reword it with a number of other pronouns, but it depends on context and whom one is talking to.

"Hey, guys!" in reference to a mixed group is perfectly acceptable. "That guy" in reference to someone you do not know the gender of is also perfectly acceptable. Directly referring to a known female as "guy" is less so.

In phrases like those the word "dude" is more common than "guy", and it is frequently directed at women.