How do gamma rays decay?
Gamma rays don't really decay. They get absorbed, and most often the energy is re-emitted as a greater number of lower-energy photons.
This is happening all the time in stars like (and including) the Sun. The energy that's directly given off from the fusion of hydrogen to helium comes as gamma rays. These gamma rays are absorbed by surrounding particles which are thereby excited to a high energy state.
Generally the particle does not directly return to its previous state in one step. It will make "stops" at multiple intermediate states that were jumped over during the initial absorption. So we get multiple photons re-emitted, but the individual re-emitted photons have only part of the energy of the initial gamma ray. It takes all the re-emitted photons to account for the energy of the one gamma ray.
Repeating this process many times over a distance of hundreds of thousands of kilometers eventually leads to the many photons of ultraviolet, visible and infrared light that finally emerge from the star.