At the dawn of the Crap Times, those with the means went underground. And those in the Silicon Valley of the Old World certainly had the means. They dug out underground complexes and bunkers where they could weather the storm. And so they did.
In time, as it became clear the fallout would take generations to clear out, not mere years, they began to explore. For many years, they were still at the height of technology, and used it to find other bunkers, other people hiding out beneath the ruined city. They tunneled further, slowly connecting their hideouts into a sprawling city beneath the city.
Lacking predators or need for muscle mass, the denizens of these bunkers slowly shrank into large-eyed, thin and pale caricatures of the humans they once were. They adapted to the underground, slowly becoming able to see in the darkness, requiring less food, and living much longer.
As resources continued to run out, their technology started to fail. Soon, all vestiges of the high-tech founders of the complex were gone, leaving behind only a culture of curiosity and the desire to solve problems through invention.
This curiosity has led the Gnomes to take the initiative in the Now What?, and the cultures of Gnomes are based largely on where they count their home now that the surface has opened up to them.
Forest Gnomes
The Forest Gnomes, overjoyed at being able to dwell on the surface of their ancestors, immediately moved back out into nature once they emerged from the bunkers. They take pleasure in the natural world and seek to make up for lost time in connecting with the flora and fauna that surrounds them.
Forest Gnomes also have a knack for simple magic, which may be what drew them to the forests. Their ability to create minor illusions is a useful survival trick in the still-dangerous wilds of the wastes.
Rock Gnomes
The Rock Gnomes took up residence in the city ruins, glad after all these years to be back under the sky. They revere their ancestors greatly, and are pleased to be walked the same earth as the humans whose foresight allowed their birth.
As part of their reverence of the humans before the Big Wake Up, the Rock Gnomes pursue technological means to whatever end they can. Most are adept at tinkering and clockwork, and frequently try to recreate the legendary technology of the past. Some even carry ancient, broken things left behind in the bunkers after all these years, hoping to one day master the ability to fix them and unlock their secrets.
Deep Gnomes
The Doep Gnomes, unlike the other gnomic cultures, found they loved the underground. When the others went up to the surface to find other survivors and explore the wastes, the Deep Gnomes stayed in the bunkers, jealously guarding the lands that enabled their survival for so many years.
They are more isolated than other cultures, and more insular, but Deep Gnomes aren't misanthropic. Some individuals even like to go out into the world and travel (though they are considered strange by their peers), but the society as a whole just gets a little nervous out in the open air. The bunkers and complexes were good enough for their parents, and their parents' parents, and it's good enough for them, too.