Spectral properties of photoinduced fiber Bragg gratings written in germanosilicate fibers and subjected to hydrogen loading at a pressure of 150-170 MPa have been studied. It was observed, for the first time to our knowledge, that hydrogen dissolution in the glass network at such high pressures leads not only to a Bragg wavelength shift but also to a considerable alteration of the grating reflectivity. The relative magnitude of the latter effect is independent of the dissolved hydrogen concentration and is defined mainly by the core glass properties and the UV-irradiation conditions. The alteration of the grating reflectivity observed experimentally is explained by increased solubility of molecular hydrogen in UV-exposed germanosilicate glass.