The effect of gamma-irradiation of the fluorine-doped silica depressed cladding graded-index fibre (DCGIF) has been researched. New mechanism of the non-bridging oxygen hole centers (NBOHC) defect formation, conditioned by different mobility of diffusion counter flows of fluorine and oxygen at the core-cladding interface, has been proposed. Using silica tubes with 0.02 wt% of OH groups and hydrogen in free state in MCVD-technology of DCGIF fabrication completely eliminates the NBOHC absorption at the wavelength of 630 nm. However, gamma-irradiation of such fibres results in anomalously high attenuation level in the visible range of optical spectrum. It was established that irradiation of DCGIF leads to light scattering which is probably due to the appearance of optical inhomogeneities in the glass matrix.