This study was a randomized, double-blind, phase 3 clinical trial which determined that locally advanced and metastatic Gastric or Gastrosophageal Junction (G/GEJ) patients with high PD-L1 levels exhibited longer survival with anti-PD-1 therapy comparatively to G/GEJ patients with low PD-L1 levels. The study enrolled 54 G/GEJ patients with locally advanced or metastatic cancer and variable PD-L1 expression. 35 of these patients were treated with Sintilimab in addition to platinum-based chemotherapy, and 19 were treated with a placebo and platinum-based chemotherapy. High expression was characterized by a PD-L1 combined positive score (CPS)≥10, while low expression was characterized by (CPS)<10. Time in months of overall survival for patients with (CPS)≥10 was evaluated at a 95% confidence interval of 0.03-1.09 and a 19% hazard. The median time of survival was evaluated to be 23.18 for those treated with anti-PD-1 immunotherapy and chemotherapy, but only 4.40 for those with a placebo and chemotherapy. Comparatively, patients with (CPS)<10 were evaluated at a 95% confidence interval of 0.30-1.48 with a 66% hazard. Those with anti-PD-1 treatment had median overall survival of 13.30 months while those without had a median survival of 12.67 months. In conclusion, this clinical trial strongly depicts longer instances of survival in patients with locally advanced or metastatic G/GEJ cancer and high PD-L1 levels who are treated with anti-PD-1 immunotherapy in addition to platinum-based chemotherapy.