Gamma radiation is actually gamma photons.
Sometimes, an alpha or beta decay leaves the daughter nucleus in an excited state.
It turns out that just like atoms, nuclei also have quantized energy levels. Because the nuclear forces in the nucleus are so strong, the gaps between the nucleus’ energy levels are measured in MeV. So when an excited nucleus de-excites, the transition from the excited energy level to the ground level produces a gamma photon.
So during a gamma decay, there is no change in the nuclear composition. It simply changes from an excited nucleus (denoted by *) to a ground state nucleus, releasing the energy in a gamma photon (denoted by).