The Gd-doped CeO2 (GDC) diffusion barrier between the zirconia-based electrolyte and the LSCF-based cathode in a solid-oxide fuel cell (SOFC) is essential for preventing the formation of Sc2ZrO3 as a secondary phase. However, the reaction between ceria and zirconia at high temperatures (>1300 °C) impedes the formation of a highly dense GDC layer. Herein, we present a cost-effective approach for fabricating a highly dense GDC diffusion barrier layer at lower sintering temperatures by filling the pores of the GDC skeleton using a gelatin or polyvinylpyrrolidone (PVP) solution as a chelating agent for GDC cations. We investigated the interaction between the cations and the chelating agent molecules and its effect on the diffusion barrier. In addition, the flow behavior of the pore-filling solution was evaluated to determine its penetrability. The proposed method yielded a pore-filled GDC (PF-GDC) interlayer with enhanced density at 1000 °C, a remarkable 250 °C below the conventional sintering temperature for a porous GDC interlayer. The effectiveness of the PF-GDC was investigated by analyzing the performance of electrolyte-supported cells (ESCs) and anode-supported cells (ASCs). On ASCs, the observed peak power density at 800 °C was enhanced 1.5-fold, from 1.91 W⋅cm−2 (porous GDC sintered at 1250 °C) to 2.61 W⋅cm−2 (PF-GDC sintered at 1200 °C). These findings highlight the potential for pore-filling methods to improve the performance of SOFCs.