I'm a dude. Now, let's explain my gestures.
With my close male friends I normally request a brofist and say a verbal greeting of some kind.
With my close female friends it's always a hug. Now, I made a lot of new friends in January, that all share my love for rock and alternative music, and they all receive hugs when I meet them. I really like hugs, and the more I like the person, the better I like the hug.
When I'm talking to a few people at once, and if one is a particularly quiet, shy or awkward girl, I'll do a friendly smile and a wave randomly while talking to the people. This usually earns me a smile.
I use "the nod" like it's described in the OP: up means "hello, I know you, and this is a light-hearted, just passing by kind of acknowledgement". Down means "I respect you", and is used for male authority-figures like teachers, along with general people that I don't know personally, but respect. I've never seen a girl use the nod, if I'm correct. I think my ex-girlfriend used it once in response to my nod. I felt awkward after realizing I had "nodded" my girlfriend.
And now I'll end the post, before I fulfil my urge to write a giant wall of text about how much I miss the times I had with my ex-girlfriend...