We have used atomistic computer simulations to study anion diffusion coefficients and the response to swift heavy ion irradiation of Gd₂Ti_xZr_2−xO₇ pyrochlore for x values of 0, 0.5, 1, 1.5 and 2. In Gd₂Ti₂O₇, a thermal energy deposition per unit length of 12 keV nm⁻¹ results in a cylindrical amorphous track of radius 2 nm in good agreement with experiment. The volume swelling of the track is 4%, which suggests that it is partially amorphous. Gd vacancies and Ti interstitials along with cation exchange play a role in damage accumulation in the titanate. In sharp contrast, Gd₂Zr₂O₇ does not swell and merely transforms from pyrochlore to fluorite under the same conditions. The activation energy barrier for oxygen hopping by the vacancy mechanism in these pyrochlores is 0.26–0.44 eV. The radiation tolerance of gadolinium zirconate pyrochlore is related to the efficient annihilation of cation Frenkel pairs, the low energy cost and negligible volume expansion associated with the resulting cation exchange, and efficient annealing of anion sublattice damage.