Gizmo is an overweight and manipulative man who runs Gizmo's Casino on the west side of Junktown. He is a fat slob of a bully who dreams of running the entire town unhindered by do-gooders like Killian Darkwater. He has tried to eliminate Killian several times already, but also managed to keep his own hands relatively clean in each attempt. Killian is looking for a way to get proof that Gizmo is behind the attempted assassinations. Gizmo is a genuine lowlife who thinks he's the real power in town. Disobey his order, and he will have you killed.
Gizmo often has the Skulz do his dirty work. His bodyguard, a master martial artist named Izo, rarely leaves his side. He partakes in numerous illicit businesses on the side, and has several trading partners in the Hub. Due to his aspirations of assuming power in Junktown, he is portrayed as the nemesis of the town's mayor, Killian Darkwater. With Killian gone, Gizmo would be able take over the town unimpeded.
After death, Gizmo's corpse is left where it is, keeled over on his desk. If the Vault Dweller visits the casino some time later, his corpse appears changed, rotting, with insects flying above it. Should the Dweller return a long while later, Gizmo's corpse will no longer be present.
In early designs, the results of dealing with Gizmo and Killian were intended to be less black and white: in the cut endings, a victorious Gizmo would turn Junktown into a flourishing trading center, albeit for entirely self-serving reasons, whereas a victorious Killian would gradually become a well-intentioned tyrant, grinding the town slowly into the dust. However, this original intention never made the final cut.
Gizmo was named after Timothy Cain's pet skunk he had when he was a kid.
The following is based on unverified behind the scenes information and has not been confirmed by canon sources.
Gizmo and Izo's dynamic are likely a reference to Mr. X and Shiva from the Streets of Rage franchise. Gizmo is similar to Mr. X in how he is a crime lord with a master martial artist as his bodyguard. Mr. X also does not move from his throne while the player fights Shiva and simply taunts them instead, while Gizmo does not move from his desk throughout the whole fight, only sitting stationary and shooting at you.
End of information based on unverified behind the scenes information.