Gamma radiation can interact in a number of different ways with matter. the 3 main phenomena are
1-photoelectric effect: a photon hits an electron in an atom and disappears giving to the electron all its energy ($E=h\nu$, where $h$ is a constant and $\nu$ the frequency of the radiation).
For this to happen the energy of the photon must be exactly equal to the amount required for the electron to transition to another bound state(ie. still being under the influence of the atomic nucleus), or more that the amount required to completely free the electron from the atom. In this case we have photoionization
2-compton scattering: basically scattering between photon and a weakly bonded (ie. is on the outmost levels of the atom) electronic atom which frees the latter from the atom. In general this scattering can happen in the presence of any electron that can be considered approximately free.
3-couple creation: a (high energy) photon decays into an electron and a positron, these beta particles can then ionize atoms. Note that for this to happen the energy of the photon must be greater or equal to the mass energy of an electron and a positron (1.02 MeV)