Gd2Ti2O7 and Gd2Zr2O7 pyrochlores were irradiated with swift heavy ions in order to investigate the effects of the chemical composition on the structural changes induced by high electronic excitation. The XRD results show that the structural modifications induced by irradiation with 93 MeV Xe ions are strongly dependent on the sample composition: Gd2Ti2O7 is readily amorphized, whereas Gd(2)Zr(2)O7 is transformed into a radiation-resistant anion-deficient fluorite structure. Atomistic simulations with the second-moment tight-binding QEq model allow us to calculate the lattice properties of both Gd2Ti2O7 and Gd2Zr2O7, and also to quantify the degree of covalency and ionicity in these compounds. These calculations clearly show that Gd2Ti2O7 is more covalent than Gd2Zr2O7, thus confirming that the amorphization resistance can be related to the covalent character of insulators.