This study aims to bridge this gap by evaluating the cost‐effectiveness of pembrolizumab in combination with trastuzumab and chemotherapy (PEM + TRAS + Chemo) compared to trastuzumab plus chemotherapy (TRAS + Chemo) as a first‐line treatment for patients with advanced HER2‐positive gastric or GEJ adenocarcinoma. The gene discussed is ERBB2; the disease is gastroesophageal junction adenocarcinoma.