In this study, we unexpectedly found that low-level Ca modification of Gd-doped ceria (GDC) electrolytes results in reduced electronic conductivity and enhanced grain boundary conductivity, while the Ca-modified composite cathode demonstrates improved oxygen adsorption performance. The results reveal that 0.5 mol% Ca doping can reach the highest total conductivity, while the ionic conduction activation energy is reduced to 0.831 eV. Schottky model analysis reveals a decrease in effective grain boundary width from 4 nm to 1.6 nm and a ∼90% reduction in oxygen-ion migration energy. Electrolyte-supported cell tests further showed a reduced electronic conduction activation energy (from 1.719 to 1.368 eV), an OCV increase of 0.119 V at 600 °C, and ∼15.6% lower cathodic polarization resistance. Consequently, the Ca–GDC electrolyte-based anode-supported cell enables a peak power density of 1.152 W cm⁻²@ 700 °C, a 30.9% improvement over undoped GDC, with a low degradation rate of 0.045% h⁻¹. These results demonstrate that minor Ca doping endows GDC with excellent conductivity and electrochemical performance, highlighting its potential as an electrolyte for intermediate-to-low-temperature SOFCs.