Magnetic and transport properties of polycrystalline Gd_1-xSr_xCoO_3-δ (x = 0.10−0.70) annealed under the oxygen pressure of 165 atm at 500 °C are systematically investigated. Cluster-glass behavior is observed in the low doping range, while a ferromagnetic transition occurs at the high doping level. Transport measurements indicate insulator-like behavior for the samples with x ≤ 0.30, an insulator−metal transition around x = 0.35, and metallic behavior for higher x samples. However, the large oxygen deficiency leads to a re-entrance of insulator-like state for the samples with x ≥ 0.60. The annealing procedure under the high oxygen pressure at high temperature can diminish the oxygen deficiency and leads to restoration of the metallic state. The small radius of the Gd³⁺ ion results in less conduction and lower T_c compared to those of La_1-xSr_xCoO₃ and Nd_1-xSr_xCoO₃ as a result of the large structural distortion and the stability of the low-spin state.