Immune checkpoint inhibitors are new targeted treatments that harness the body's immune system to attack cancers. Drugs that are most extensively used among checkpoint inhibitors inhibit the PD-L1 or PD-1 (programmed death 1) ligand or receptor pair and are currently approved for many cancer indications. In gastric or gastroesophageal junction adenocarcinomas one inhibitor, pembrolizumab has regulatory approval for PD-L1 positive carcinomas. This meta-analysis investigates available data on the efficacy of PD-L1 or PD-1 inhibitors as a class in gastric or gastroesophageal junction adenocarcinomas. The literature was reviewed to identify clinical studies that included arms with PD-L1 or PD-1 inhibitors as monotherapy in gastric or gastroesophageal junction adenocarcinomas. Relevant patient characteristics, outcomes, and adverse effects were recorded. Summary estimates of response rates (RR) and survival were calculated using a random or fixed effect model, depending on heterogeneity. Six studies with a total of 1068 patients were included in the analysis. The summary RR was 10.63% (95% confidence interval (CI) 5.36–15.89%). The summary disease control rate (DCR) was 28.11% (95% CI 24.60–31.63%). Summary progression-free survival (PFS) was 1.59 months (95% CI 1.24–1.94 months). Summary overall survival (OS) was 5.72 months (95% CI 0–12.19 months). A subset of patients derived long-term benefits as seen in other cancer locations. The adverse effect rate was low and consistent with that in other disease locations. Low efficacy of immune checkpoint inhibitors as a class in gastric or gastroesophageal junction adenocarcinomas is observed in this analysis and stresses the need for effective biomarker use for the identification of most probable responders.