The KEYNOTE-811 clinical trial revealed that the combination of pembrolizumab with trastuzumab and chemotherapy as a first-line treatment for metastatic HER-2-positive gastric or gastroesophageal junction adenocarcinoma significantly improved progression-free survival (PFS) compared to placebo, particularly in patients with PD-L1 CPS ≥1. This evidence concerns the gene ERBB2 and gastroesophageal junction adenocarcinoma.