The influence of Gd doping at La-site on the electrical transport properties and the colossal magnetoresistance of La0.7−xGdxSr0.3MnO3 (x=0.00, 0.10, 0.15, 0.20, 0.30, 0.40, 0.50, 0.60, and 0.70) is studied. The experimental results indicate that the transport properties exhibit abnormal behavior under high doping condition. Forx=0.50, we find that a transition from metal to insulator occurs after the occurrence of insulator-metal transition nearTc, which seldom occurs in ABO3 structure. For samplesx=0.60 and 0.70, it exhibits insulator behavior far aboveTc. These abnormal behaviors are attributed to different magnetic background, i.e. the system undergoes a transition from long range ferromagnetic order to the cluster-spin glass state and further to antiferromagnetic order.