The thermal properties (thermal conductivity κ(T), thermal diffusivity α(T), thermoelectric power S(T) and thermal expansion dL(T)/L) of melt-processed Gd-based bulk superconductors are measured in the ab-plane and along the c-direction and compared with those of the Y-based system. The ab-plane κ_ab(T) values of the Gd-based system show a characteristic large enhancement below the superconducting transition temperature T_c, which is suppressed by the magnetic field up to 10T applied perpendicular to the ab-plane. The results on the thermal properties suggest that GdBaCuO bulk crystals make a clean system similar to the Y-based system and are in marked contrast to the Sm-based system, where Sm-Ba substitution occurs.