It's often a symbol of friendship or lurve. Unless the person has pointy teeth..then it's quite painful.
People have been known to respond to gnaws with : chew, nibble, counter-gnaw, various yells, and death glares.
It's also a wonderful way to greet a person, as shown below. (screennames have been changed so there :p ) taken from real conversations.
Moony: ^-^ I feel lurved
Hell: ^^ you are
N-chan: Ah!! No teeth!
B-chan: *Blink, blink* ...Rawr?
N-chan: ... *smacks you with pillow*
2.Gnaw is a way to say that was completly useless.
3.Gnaw is a way to express not wanting to do something
4.Gnaw is a form of acknoledgement.
2."Hey i like cheese""tch, ur gnaw"
3" honey go clean up the garage. please" "GNAW! im too tired, I will do it tommorow."
4." hey man you want to go to the halo tournament?" " GNAW!. Why wouldn't I"