It came from the movie how high, it means for 2 people to be gunning
on eachother back and forth. Also after each insult everyone around yells "Get em"
1: You so ugly, your birth certificate was sent to your parents with a sorry letter from the condom company
Everyone: Get em!!!!!
It means to insult someone repeatedly (verbally-usually using things about them, yo mama jokes, etc)
Damn Joey, you was straight gunnin on his ass