A gesture meaning "your dick". The gesture is made with your thumb and index and is aimed at the receiver in a "U" position, and it represents the length of a small penis, therefore making the gesture an insult; especially funny to use when you're driving down the road and you see some douche bag you hate, so you honk the horn and give them The Gunnat.
Adam: "Hey, isn't that Luke walking down the street?"
Christian: "Yeah! Oh my God, he's such a fuckin' douche, he fucked my girlfriend last week and got her pregnant. God I hate that piece of shit!"
Adam: "Dude, give him the gunnat!"
Christian: (Honks the car horn and gives Luke the gunnat) "Hey, fuck you!"
Adam: Ha ha ha ha, he's such a faggot!"