The superconducting properties of the RuSr₂GdCu₂O₈ ceramic system depend strongly on the synthesis conditions and the ionic substitutions. In this work, we studied the structural and electrical properties of the Ru_0.9M_0.1Sr₂GdCu₂O₈ system, with M = Zr, Mo, and Mn. The samples were prepared by solid-state reaction at ambient pressure in air, using temperatures between 980 °C and 1020 °C. X-ray powder diffraction patterns indicate that all samples crystallize in a tetragonal symmetry (S.G. P4/mmm, No. 123). The structural data of each sample was refined by the Rietveld method, showing that the Cu-O (1) and Ru-O (1) bond lengths vary with the substituted ionic radii of Zr, Mo, and Mn ions. Electrical resistance measurements indicate that the samples annealed in flowing oxygen at 1050 °C and 1055 °C for 5 days exhibit a semiconductor like behavior for M = Mo and Mn, whereas the samples with M = Zr and Ru show superconducting behavior.