|Side Quest: Optimize the powerplant|
|Finish Solve the Gecko powerplant problem quest.|
|Speak with Festus.|
|Go to Vault City and speak with McClure.|
|Go to Vault 8 and insert the disk in the main computer.||Persuade McClure to optimize the disk for you.|
|Return to Gecko and install the disk in the computer.|
|Reward: 2500 XP|
Speak with Festus; he will tell the Chosen One to optimize the plant and give them the optimization disk. Afterwards, proceed to Vault City, speak with McClure, and run the disk on Level 3 Vault Computer. Return to Gecko and use the disk on the computer console in the powerplant to optimize the plant. Alternatively, one could talk McClure into optimizing the disk for them.
Gordon in Gecko mentions that, if the powerplant is optimized, Gecko can trade excess electricity to Vault City for medical supplies. But Vault City doesn't care: if the powerplant is optimized, Vault City invades Gecko, takes control of the city and enslaves the ghouls.
There is an option to present McClure with a holodisk showing the power the plant could produce if optimized, and the player character can have McClure offer to speak to the Vault City council about a trade agreement with Gecko, but this does nothing to change the result due to a scripting glitch. Once the plant is optimized, Gecko's fate is sealed. Even changing the outcome of Vault City's dealings with NCR does nothing to change Gecko's ending.
Speaking with McClure is not necessary to download the optimization data, and no one will care if the player character did it without McClure's permission.