Gamma decay is the easiest! It is the emission of a gamma-ray with A = 0 and Z = 0 leaving the daughter nuclides numbers unchanged.
Gamma decay occurs when nuclides have too much energy. This can be from heating, absorbing light, or perhaps leftover from a previous decay process. In order to release this energy, the nuclide releases a high-energy photon, a gamma-ray.
A gamma-ray has extremely low mass and therefore has a mass number of zero and an atomic number of zero. A parent nuclide with excess energy is denoted with an (m). For example, uranium-239m, a parent nuclide, would decay via gamma emission into uranium-239 and a gamma-ray.
Gamma rays are useful too. Airport security use them to look inside our luggage, and doctors use them to sterilize their equipment and take X-ray images of bones.