Electrical properties of 10 mol% Gd2O3-90 mol% CeO2 (GDC10) and 20 mol% Gd2O3-80 mol% CeO2 (GDC20) ceramics, sintered in various temperatures from 1100 to 1500 A degrees C for 2 h, have been investigated by 2- and 4-electrode impedance measurement methods. The sintering temperature was found to have a significant effect on electrical properties of the grain boundary medium. A brick-layer model-based analysis of the experimental data of grain boundary medium was carried out for the estimation of proportion ratio between grain dimension and grain boundary thickness. Impedance spectra, obtained by 4-electrode method, were also examined by numerically calculating the probability density function of distribution of relaxation times of charge carriers. The latter method revealed time domain behavior of the system, which allowed more accurate evaluation of the most probable relaxation times. GDC10 samples, sintered at 1400 and 1500 A degrees C, showed a particular relaxation behavior of charge carriers in grain boundary medium. Two distinct peaks of the imaginary impedance (both associated to grain boundary) were detected, which suggest that grain boundary medium in these specimens may consist of two phases with different electrical properties.