وعليكم السلام ورحمة الله وبركاته
gamma rad has many sources on the earth
like the relaxation that occur after the decay by beta mode
in nuclear rxn when the nucleus absorb neutron in many time gamma were emitted
gamma emitted when any of the nucleons change its energy level like x ray in the atom
if you need more on this topic you have to read nuclear book like" nuclear physics in nutshell"good luck