Under what circumstances would a guy high five and/or fist bump a girl, when they've been flirting back and forth previously? Is it just a friendly gesture? Is it a casual way of flirting? Or would it be more like him saying, "We are friends, buddy. Nothing more."

How old are these people, in what context are they flirting,
But flirting is flirting, it could mean anything or nothing, even the context of how it is done can make a difference.

My co-workers, both male and female, do it as a friendly gesture. My friends outside of work don't really do it at all.

But like chuck said, it all depends on the context

Sometimes high fives and fist bumps are a guys lame way of flirting with you

Depends on:
The circumstance,
The guy,
The signals you put out there,
The people around you

The only SURE thing is when a guy asks you out, you can be sure he's interested in you. All the other flirting is open for interpretation.