Ojiu0u4,
From what I can make out from the Wiki quote below, if a Gamma burst smaller than the mesh in the microwave hits the microwave then radio wave ingress will happen.
[quote from wiki, Gamma rays typically have frequencies above 10 exahertz (or >1019 Hz), and therefore have energies above 100 keV and wavelengths less than 10 picometers (10-12 meter), which is less than the diameter of an atom. However, this is not a hard and fast definition, but rather only a rule-of-thumb description for natural processes. Gamma rays from radioactive decay are defined as gamma rays no matter what their energy, so that there is no lower limit to gamma energy derived from radioactive decay. Gamma decay commonly produces energies of a few hundred keV, and almost always less than 10 MeV.]
I'd put my money on a lead lined steel box with earth strap being the best mode of gamma radiation protection for radios.
Not being an RF genius by any degree I stand to be corrected by anyone with a better understanding and without prejudice
Wulfshead