|Japanese Voice||Takehito Koyasu|
|English Voice||Mike McFarland|
Appearance-wise, Gastro is one of the more flamboyant-looking characters in the series. He has dark, spiked-up hair and furrowed brows, and tends to wear classy clothing. Strangely, when his gun is in his mouth, his eyes tend to go unfocused and point in different directions.
Gastro is a professional hitman who takes pride in his skills. Although he is a capable professional, he has a sense of honor, and considers cruelly poisoning a class of middle schoolers beneath him. However, he has one particularly odd personality quirk: he considers the flavor of his gun to be exquisite, and is often seen with it jammed into his mouth (occasionally covered in soup).
Gastro first appears as an assassin to stop Nagisa and the others from retrieving the antidote from Takaoka.
When encountering part of Class E in the theater, he easily spots every single of their locations, shooting at them, though they luckily survive due to taking cover. Though the Class E feels hopeless, Korosensei comes to the rescue and distracts Gastro with his unique, invincible form, while giving the remaining Class E members directions to swap positions to confuse Gastro.
Though confused, Gastro finally believes to have located an E-Class student, waiting for their movement. As the "student" moves, it only turned out to be a decoy, made during the attack itself. Chiba and Hyami work together to disarm Gastro, leaving him to be tied up by the rest of the class.
After Takaoka is taken down by Nagisa, Gastro and the other assassins have actually escaped using their assassination knowledge. Though hired by Takaoka, Gastro explains to them that they actually swapped the Takaoka's virus formula with a replica that only works for three hours, but doesn't end up killing the victims. He explain further to the E-Class members that losing his reputation of being an excellent assassin would be better off than being worth a bounty for killing a class of students and the trio make a discussion before the class arrived on this island and they decide to maintain their comply to Takaoka through swapping virus formula unnoticed. He then leaves on a helicopter, explaining that he'll kill Class E only if they're worthy of it until the class are put on his "Hitman list".
Later the trio (Gastro, Smog and Grip) makes their appearance again at School Festival where he also tastes the acorn noodle with his gun. They are invited by Koro-sensei to visit the Class E Classroom Cafe to have a taste of the Acorn Noodle made by Class E.
Marksmanship: Possibly due to formally being a military gunman, Gastro has the ability to know where his targets are and their precise movements. Combined with his highly accurate aim, he is one of the best marksman to date.
Enhanced Senses: Gastro has heightened senses which he utilizes in his shooting. Most notably, his hearing which is sensitive to the point where he can hear light breathing and can even determine the age range of the person.