He used to be a member of American Mafia, going under the name Gaff. He at some point came into conflict with Go Shijima and killed one of his best friends, Ethan Woodward.
Alongside his "little brother" 018, he started a ring of devices that cause Heavy Acceleration surges, causing their users to do whatever they wish of them. He first fought against Drive to cover one of his clients. After fighting Drive to a standstill, he distracted the Rider with a spray of bullets before fleeing the scene.
He later corners Shinnosuke and Go in a hail of gunfire when the two Riders and Genpachiro discover that he and his client were not the same person. Gunman fought Drive as Type Speed. However, Go soon appears as Kamen Rider Mach and beats back the Roidmude before landing a Kick Macher on him. However, Gunman was saved by Medic, who brought him back to full health. Gunman then introduced his "younger brother" to the Roidmude heads.
Mashin Chaser later made Gunman and 018 his guards. Despite this, the two continued their black market business. However, Go was able to infiltrate the two Roidmude siblings' base. The three Roidmudes attempted to pursue him but Drive and Kiriko soon arrived. Go then transformed into Mach and destroyed the two Roidmude brothers.