We have studied the gamma-radiation induced effect on the Bragg peak position in fiber gratings. For the first time, to the best of our knowledge, a short-wavelength shift was observed for the Type I gratings written in an intrinsically photosensitive fiber. Based on the kinetics of the peak shift observed under irradiation and during post-irradiation annealing, we propose that two different types of defects are generated by ionizing radiation. One type is responsible for the refractive index increase during irradiation, while the creation of the defects of another type results in the index decrease. The concurrence of the two types of defects can be a way to obtain high stability of the gratings under irradiation.