I think you did an excellent job at the mouth and Hiru's panic. I'm terrible at mouths still so um, no help here. Although thinking about it, I guess a gag happens more in a constricted throat and jaw, and travels to a pushed up bottom lip, which meets the tongue because the tongue is trying to push out whatever is in there. *shrug*
Now I want to try draw someone gagging but I know I'll cry tears of frustration because it seems very hard.