Gd1-xLaxBaCuFeO5+ polycrystalline ceramics have been prepared by a sol-gel method combined with the traditional ceramic processing. Analysis of the microstructure and phase composition reveals that the pure GdBaCuFeO5+ phase can be obtained even after the substitution of 50% La at Gd-site. The band gap can be tuned from 1.47 to 1.36eV by La substitution, resulting in a significant increase in the electrical conductivity. In addition, the total thermal conductivity can also be suppressed by the substitution of La at Gd-sites. A ZT value of 0.02 at 1023K is achieved in the 50% La-substituted samples, which is over 20 times higher than that of the pure GdBaCuFeO5+ sample.