Inclusion Criteria:

- Adult patients, >/= 18 years of age

- HER2-positive metastatic adenocarcinoma of the stomach or gastroesophageal junction

- Measurable or evaluable non-measurable disease as assessed by the investigator
according to RECIST v1.1 criteria

- Eastern Cooperative Oncology Group (ECOG) performance status 0 or 1

- Life expectancy >/= 3 months

Exclusion Criteria:

- Previous cyctotoxic chemotherapy for advanced (metastatic) disease

- Evidence of disease progression documented within 6 months after completion of prior
neoadjuvant or adjuvant cytotoxic chemotherapy, or both, or radiotherapy for GEJ
adenocarcinoma

- Previous treatment with any HER2-directed therapy, at any time, for any duration

- Previous exposure to any investigational treatment within 30 days before the first
dose of study treatment

- Radiotherapy within 30 days before the first dose of study treatment (within 2 weeks
if given as palliation to peripheral bone metastases, if recovered from all
toxicities)

- History or evidence of brain metastases

- Clinically significant active GI bleeding (Grade >/= 2 according to NIC-CTCAEv.4.03)

- Other malignancy (in addition to GC) within 5 years before enrollment, except for
carcinoma in situ of the cervix or squamous or basal cell carcinoma of the skin that
has been previously treated with curative intent

- Inadequate hematologic, renal or liver function

- Pregnant or lactating women

- History of congestive heart failure of any New York Heart Association (NYHA) criteria

- Angina pectoris requiring treatment

- Myocardial infarction within the past 6 months before the first dose of study drug

- Clinically significant valvular heart disease or uncontrollable high-risk cardiac
arrhythmia

- History or evidence of poorly controlled hypertension

- Baseline left ventricular ejection fraction (LVEF) value < 55%

- Any significant uncontrolled intercurrent systemic illness

- Positive for hepatitis B, hepatitis C or HIV infection