From Dragon Ball Encyclopedia, the ''Dragon Ball'' wiki
Super Boo opens his mouth and charges a pink energy sphere with additional streaks of black and pink electricity. He then fires the energy sphere from his mouth in the form of a narrow, pink wave of energy at his opponent, inflicting a high amount of damage.
Dragon Ball Z
Super Boo used this attack in his battle against Gotenks in his Super Saiyan 3 form. After Super Boo knocked Super Saiyan 3 Gotenks into a city building, Super Saiyan 3 Gotenks attempted to attack Super Boo. Super Boo counterattacked him with Gack!, however, injuring Super Saiyan 3 Gotenks and laughing at the result. Humilated, Super Saiyan 3 Gotenks blasted Super Boo with his Revenge Cannon, injuring the Majin and making him fall to the ground.
Super Boo also used this attack twice in his battle against Son Gohan in his Ultimate form. The first time was when Ultimate Gohan had easily dominated Super Boo and threw him into a mountain. Ultimate Gohan charged after Super Boo, but the Majin emerged from the rubble and countered with Gack!, which Ultimate Gohan deflected.
The second time was when Ultimate Gohan elbowed Super Boo in the face and kicked him into a mountain. Enraged, Super Boo powered up, causing the rubble around him to explode into many rocks that were fired at Ultimate Gohan, who managed to destroy every rock near him. Super Boo rode on one rock and attempted to fire Gack! again, this time at point blank range. Ultimate Gohan pushed the blast back into Super Boo's mouth and caused an explosion within him that injured the Majin.
It is named Ill Flash in Dragon Ball Z: Budokai 3 and is fired more like a sphere.