Human epidermal growth factor receptor 2 (HER2) is responsible for the pathogenesis and poor outcomes of several types of cancers, including advanced gastric and gastroesophageal junction cancer. Molecular-targeted drugs on the other hand, such as trastuzumab, prolong overall survival and progression-free survival in HER2-positive gastric cancer. The purpose of the case report is to evaluate the impact of delivering trastuzumab in advanced gastric cancer with concomitant HER2 mutation and amplification.