<p>This research investigated the evolution of structure and vacancy defects caused by Nb ion substitution at the Mn site on the physical properties of gadolinium manganite (GdMnO₃) materials synthesized via the solid-phase reaction method. XRD analysis confirmed the single-phase structure of the synthesized GdMn_1-<i>x</i>Nb_<i>x</i>O₃ ceramics and revealed lattice distortion resulting from the replacement of Mn³⁺ with Nb⁵⁺ ions. SEM results demonstrated a correlation between the grain size of the synthesized GdMn_1-<i>x</i>Nb_<i>x</i>O₃ ceramics and the vacancy concentration. XPS analysis indicated that Nb⁵⁺ substitution modified the oxidation state of Mn and influenced the concentration of oxygen vacancies; notably, the dominant charge compensation mechanism varied with different Nb substitution levels. Positron annihilation experimental results indicated that Nb⁵⁺ substitution could affect the open volume and concentration of vacancies in the Gd_1-<i>x</i>Nb_<i>x</i>MnO₃ system. Temperature and magnetic field dependent magnetization measurements showed that Nb⁵⁺ ion substitution enhanced the magnetization. The obtained results indicated that the properties of GdMnO₃ system could be optimized by introducing vacancies and converting the Mn³⁺ to Mn²⁺.</p>