The word "guy" is a noun and originally meant "boy or man". Nowadays, it's used when someone speaks to a group of people which includes women and/or children (but it's never used to address a single female). For example, a waiter in a restaurant may ask the group (including men, women and children) at a table, "What would you guys like to order?". If the word is capitalized (Guy), it is a French name of a male (pronounced as Gee). Fifty years ago, we used to informally refer to men as a "guy" and informally refer to a women as a "gal". The term gal is not used much anymore. Some English speakers, including myself, don't like to be referred to as a guy, especially by someone younger than ourselves, so I would not appreciate a waiter using the term guy when addressing me. I suggest that the term is too informal and should not be used when a younger person is speaking to an older person, regardless of whether the person being spoken to is a male or female. I don't mind if the word is used to speak to a child or group of children or used to speak to one's friends. I'd like to know if I'm the only native English speaker who doesn't like the way the word is being used these days.

"Guy" isn't a form of address, so you'd never say "Hi guy". "Guy" is just an informal word for "person". Ex. "This guy is..." = "This person is..." "The guy over there said..." = "The person of there said..."

Jmat is correct. I just want to add a few things. In some parts of the English-speaking world, it is socially acceptable to address a group of people by saying, "Hi, guys!" You can say that to a group of men, a group of women, or a mixed group, and it is okay. But strangely, "guy" in the singular always refers to a male.