How do gamma rays ionize atoms?

Actually the same way all other radiation ionizes atoms. Radiation comes in, carries energy with it, is absorbed by an electron and that electron is expelled if no higher energy orbitals are available. Bye bye electron, hello ionized atom. read more

Gamma rays are just high energy particles with negligible mass and neutral charge, to ionize a gas you need charged particle like alpha(double charged He atoms), beta(negatively charged) which can induce a charge when they bombard the particles of gas. read more

Furthermore, x-rays/gamma-rays can ionize atoms by providing an electron with enough energy to escape the atom (which I believe is the cause of the photoelectric effect). Finally, an escaped electron can release some of its remaining energy through Compton Scattering, producing a lower frequency wave than that of the incident one. read more