In fact, gamma rays wouldn’t even reach the ground. The upper atmosphere would take the hit. It’s the ozone depletion, which leads to high energy UV reaching the ground, that is the real danger.
Of course that’s from Wikipedia, so it might not be true.
The gamma rays themselves will mostly be stopped by the atmosphere. But in the process, they’ll produce muons and other particles, some of which will penetrate deeply.
So I’m still waiting for a clearer answer to one of my questions:
When Chronos said we’d all be “flash-broiled” in seconds, does that literally mean we’d be thermally heated up to cooking temperature (or incineration) temperature? In which case, all the inanimate objects I asked about would get burnt too. Or is “broiled” (or “cooked” or any similar synonym) simply a metaphor meaning we’d suffer radiation poisoning, as dracoi suggested a few posts above? In which case, we’d all die a slow, horrible and gruesome death, like those early atomic researchers who “tickled the tiger’s tail” and lost?