Why is extremely high energy gamma radiation over 100 MeV not dangerous?

Answer 1

It is true that 100MeV and higher gamma-rays aren't very dangerous at low intensities. They are, however, dangerous at high intensities (melting, burning, fire, etc.). The reason why low intensity, high energy gammas aren't as dangerous as low intensity, low energy gammas is because at higher energies they become far more likely to interact with atoms via pair-production than they do via Compton scattering, the latter being the only absorption-type gamma ray interaction. When a molecule or an electron absorb enough energy, they sometimes become unbounded, therefore becoming ions, and it's ionization that causes the most cell damage.

Long story short: no energy absorption, no ionization, no cell damage.