Get Gizmo's confession

 is a side quest in the original Fallout.

Walkthrough
After the Vault Dweller speaks to Killian Darkwater for the first time, a man named Kenji equipped with a Colt Rangemaster hunting rifle enters the store saying "Gizmo sends his regards" and fires at Killian. Once the lone assassin has been dealt with, you will earn 400 xp, in addition to the xp you get from killing him.

Mayor Darkwater will then ask the player to obtain a confession from Gizmo so he can lock him up or take him down. If you accept, go to Gizmo's office at the back of his casino.

At his stage, you have two options: 1. You can plant the bug on Gizmo with the steal skill. 2. You can persuade Gizmo to hire you to assassinate Killian while you have the tape recorder in your inventory.

Regardless of which option you choose, return to Killian to gain your reward. Although you can perform the actions for both objectives, you can only give Killian one of the pieces of evidence. Both provide the same rewards.

Rewards

 * 500 experience points
 * The Vault Dweller can take something from Darkwaters General Store. This can be either 5 stimpaks, a doctor's bag, leather armor or a shotgun with some Shotgun shells.
 * Killian offers Take Revenge on Gizmo quest.