This randomized, multicenter, open-label study will evaluate two different doses of pertuzumab in combination with Herceptin (trastuzumab) and chemotherapy in the first-line treatment of participants with metastatic HER2-positive adenocarcinoma of the stomach or gastroesophageal junction. Participants will be randomized in a 1:1 ratio to two treatment arms. Participants in Arm A will receive a pertuzumab loading dose of 840 mg for Cycle 1 and a dose of 420 mg for Cycles 2-6, and participants in Arm B will receive pertuzumab 840 mg for all six cycles. Participants in both treatment arms will receive trastuzumab, cisplatin, and capecitabine.