Gack! (ギャー!) is a mouth energy wave notably used by Super Buu. First, Buu opens his mouth and charges a pink energy sphere with additional streaks of black and pink electricity. Then, he fires the energy sphere from his mouth in the form of a narrow, pink energy wave at the opponent, inflicting a high amount of damage.
This attack is first used by the huge Buu (Kid Buu with the Southern Supreme Kai absorbed) against the Eastern Supreme Kai, but it is countered by the Grand Supreme Kai. Kid Buu later uses it in his original form against Goku and Vegeta on the Sacred World of the Kais.
Super Buu uses Gack! during his battle against Super Saiyan 3 Gotenks. After Super Buu knocks Gotenks into a building in a city, Gotenks attempts to attack Super Buu. However, Super Buu counterattacks with the "Gack!" mouth blast, injuring Gotenks and laughing at the result. Humiliated, Gotenks blasts Super Buu with his Revenge Cannon, injuring the Majin and making him fall to the ground.
Super Buu also uses this attack twice during his battle against Gohan in his ultimate form. The first time is when Gohan easily dominates Super Buu and throws him into a mountain. Gohan charges after Super Buu, but the Majin emerges from the rubble and counters with the "Gack!" mouth blast, which Gohan deflects it before it was released. The second time is when Gohan elbows Super Buu in the face and kicks him into a mountain. Enraged, Super Buu powers up, causing the rubble around him to explode into many rocks fired at Gohan, who manages to destroy every rock near him. Super Buu rides on one rock and attempts to fire the "Gack!" mouth blast again at point blank range. However, Gohan pushes the blast back inside Super Buu's mouth, injuring the Majin.
Appearances in games
This technique is called Ill Flash in Dragon Ball Z: Budokai 2 and Dragon Ball Z: Budokai 3. It is one of Super Buu's Blast 2 in Dragon Ball Z: Budokai Tenkaichi (called Super Mouth Blast), and Dragon Ball Z: Supersonic Warriors (called Dimensional Portal Cannon). It was named "Gack!" in the Raging Blast games, Dragon Ball Z: Ultimate Tenkaichi, and Dragon Ball Z For Kinect where it is one of Super Buu's Super Attacks. It also appears in Dragon Ball Z: Battle of Z, named Guoah!
In Dragon Ball: Xenoverse, it appears under the name Ill Flash and is a sweeping Mouth Energy Wave Super Skill used by Super Buu. It can also be obtained by the Future Warrior as a reward in Parallel Quest 36: "Majin Banquet".