Killquick is a physical attack for Gohan in the Budokai video game series. First, Gohan kicks the opponent up in the air with a backflip. Then, he does a knifehand strike on the opponent, knees them, and finally kicks them down to the ground, inflicting a good amount of damage.
It is very likely that Piccolo taught this technique to Gohan, as he has an attack called Instant Killquick with the only difference between the two techniques being that the Instant Killquick is stronger than the Killquick. Gohan also uses the Instant Killquick as a teen and the similar Justice Finisher as Great Saiyaman.
Techniques similar appear under different names and are used by other playable characters in the Budokai games: the Zanku Fist, Yamcha's Wolf Slice Fist, Tien Shinhan's Jackhammer Fist Flash, Raditz's Exciting Year, Nappa's Giant Attack, Recoome's Recoome Buster Magnum, Captain Ginyu's Ginyu Strike, Frieza's Frieza Nightmare, Android 20's and Android 19's Power Break, Android 18's and Android 17's Power Strike, Android 16's Devil Crush, Mr. Satan's Hercule Critical Attack, Cell's Ultimate Attack, Videl's Leopard Shoot, Trunks' Blast Attack, Gotenks' Ultra Missile Parfait, Dabura's Hell Blow Shoot, Majin Buu's Innocence Strike, Super Buu's Vice Break, Kid Buu's Crazy Smash, Gokule's Super Finish Gokule Attk/Gokule Critical Attack, and Tiencha's Sirius Hammerfist.