Nothing much left to do here, except go talk to Festus and then go use the optimized reactor disc on their computer.
You can also try to bait an Enclave trooper by hacking into their
network. It will give you an idea of what you're up against, but if
you screw up, he'll say he's sent troops to get you. I hanged around
for a few minutes to see if the Enclave troopers arrive (though I knew
I had no chance in hell to survive the encounter), but they didn't.
Maybe they needed more time? I reloaded just in case.
You can talk again to Harold, ask him about what he does, say you
didn't want to see their city destroyed, and work your way to telling
him the story of the Vault Dweller. He'll give you something.
Btw, before I leave the subject of Gecko, there's a building with a
manhole in the north section, where you find some brainwashed ghouls
and an overgrown molerat called The Brain, who wants Cheesy Poofs from
you and plans to take over the world. Other than being another
gratuitious "Animaniacs" plug, he can give you hints on how to do the
Gecko quests if you talk to him before you finish them. I fail to see
any point in this exercise, so you may skip this area completely.
Interestingly enough, if you talk to him after you've killed the "rat
god" in Klamath, The Brain will be sad about it. This has made me
wonder if you can't find a more peaceful solution for Klamath's rat
problem by talking to The Brain first. Nope, there's no option to
tell The Brain about Klamath.
That's it. Leave and head back to the Den, via Modoc.