As an early 90’s millennial, I’ve never noticed a “gen z stare” as described in news articles like a “blank face that shows lack of social skill or ability to think”. The only times I’ve witnessed it happen and seen the older person accuse them of “gen z stare” is when the older person says something off hand or dumb but isn’t self aware enough to realize they’re being weird. Hell, I’ve given people a blank face countless times because I was taught it was better to say nothing at all sometimes. Especially when it came to talking to older people at work.
I remember when I was 16, some middle aged guy at work accused me of having no personality. In reality, I kept all conversations short as possible with him (like almost everyone in the store) because they were casually racist and misogynistic.
“Good you” is the perfect response, its just a more personal version of Hi or Hello, no need to over think it. As for the second part if you know the person hows it going can just be a conversation starter, its meant to ask what are you up to, i.e. is there a light topic we can have small talk about that isnt going to be too involved. You can respond with something along the lines of “I’m doing good, such and such happened the other day that was nice, how about you.”