Gamma-ray bursts are over quickly. A sufficiently-nearby burst might flash-sterilize half the planet, but the other half won’t feel any direct effects from it. It might feel indirect effects, but those will mostly be carried by the atmosphere, and so won’t easily cross the Equator. So the only real global risk from a GRB would have to be from one near the plane of the equator, so as to hit both the North and South alike with the indirect effects.
I have always understood that the really bad long term effect from a GRB is that it utterly strips the planet of any protective layers like the ozone and such. The burst itself is only a matter of seconds, right?
Plausible. I’ve also heard that it might produce opaque nitrogen oxides in the atmosphere, that would shade the surface. Either way, those would be atmospheric effects, which would as I said mostly not cross the Equator.
Set up a text notification for when the Sun changes it’s Facebook status.
The moon’s not going to work; try an asteroid instead: Astronomical engineering: a strategy for modifying planetary orbits
Mandatory Bruce Cockburn link: When the Sun Goes Nova
Niven pinched the idea of using the moon to detect a nova from Arthur C. Clarke, who used it in Rescue Party.