In this work new RP materials of the La2-xGdxNiO4+δ series were synthesized and their structural features, oxygen stoichiometry and transport properties were studied by X-ray diffraction, TGA, mass relaxation method and oxygen isotope exchange with C18O2 in a flow reactor. According to XRD data, La2-xGdxNiO4+δ (x = 0.0; 0.2; 0.4) are single-phase samples with a tetragonal structure. Doping with Gd resulted only in the insignificant increase of the interstitial oxygen content (δ changes in the range of 0.13–0.15). A high oxygen mobility was demonstrated: at 700 °C Dchem ~ 10−5 cm2/s (mass relaxation data) and self–diffusion coefficient D⁎ ~ 10−8 cm2/s (oxygen isotope exchange data). Such characteristics are determined by the cooperative mechanism of oxygen migration involving both regular and highly-mobile interstitial oxygen. Gd doping, however, resulted in non-uniformity of the oxygen migration routes. To describe this feature, a mathematical model has been developed including a slow exchange between neighboring oxide anions. Oxygen mobility decreases with increasing Gd content.