I am dubious that gamma ray bursts are invariably a sentence of doom. The actual mechanism is due to the destruction of the ozone layer due to nitrogen molecules formed in the upper atmosphere; these molecules would "eat" the ozone for maybe 4 - 5 years after a GRB event, but would not (in that sort of lifetime) go from one hemisphere to another. Questions I would have include
- How many civilizations might form on bodies with very thick atmospheres, far from their Suns? (Venus does not need a ozone layer to keep the UV out, and might be very habitable a few AU out.)
- How many planets might have very long rotation periods (years), so that the night hemisphere never is subjected to the daytime UV?
- Are there rotation axis directions and orbital precession constants for planets that would keep GRB radiation mostly in one hemisphere, leaving the other to develop?
- How many planets might have other special circumstances that protect their ozone (such as a lack of N2 in their atmosphere, or an ozone generating biology in their stratosphere, etc.)
I am sure that there are others, but even these I think show that, while GRB might be bad for habitability, they need not be fatal. Note, too, that if I was running a Kardashev Type III civilization, one of my action items would be to find any possible GRB progenitors and disarm them. So, in a KIII galaxy, GRB would likely no longer be a problem; maybe that would be a good way to determine the number of KIII galaxies in the universe.