ERBB2 and gastroesophageal junction adenocarcinoma: Until recently, standard first‐line treatment for unresectable advanced or metastatic human epidermal growth factor receptor 2 (HER2)‐negative gastric and gastroesophageal junction adenocarcinoma has been fluoropyrimidine plus platinum‐based chemotherapy, which results in poor median overall survival (OS) of <1 year.5, 9, 10, 13, 14, 15, 16, 17