In situ measurements of the resistance changes of carbon nanotube bundles irradiated by 60Co γ-quanta at room temperature are presented. The peculiarity of our study consists in providing the sample irradiation in a hermetic cell filled with different gases at atmosphere pressure. Here we demonstrate the results for irradiation in hydrogen, deuterium, helium and argon media. The obtained dependences of the resistance change versus irradiation dose in vacuum are presented, too. We have developed a novel two-stage mechanism, which implies defect introduction by energetic intermediate particles that are gas atoms. Preliminary calculations for Compton electrons (0.85 MeV) and atmospheric gas pressure show the high efficiency of this two-stage mechanism in comparison to conventional one-stage mechanism. It is shown that the defect introduction rate in the developed two-stage mechanism is strongly dependent on the gas atomic mass and coincides well with our experimental results.