Inclusion Criteria:

          -  Group 1, patients with treatment-naïve recurrent or advanced metastatic gastric cancer
             including gastroesophageal junction or upper part of the stomach.

          -  Group 2, patients with recurrent or advanced metastatic gastric cancer including
             gastroesophageal junction or upper part of the stomach, who were treated ≥2 times with
             palliative chemotherapy before screening.

          -  At least 1 evaluable lesion for the dose escalation part and at least 1 measurable
             lesion according to RECIST v1.1 for the dose expansion part.

          -  Eastern Cooperative Oncology Group (ECOG) performance status ≤1.

        Exclusion Criteria:

          -  Symptomatic central nervous system or uncontrolled brain metastasis

          -  Carcinomatous meningitis or its history.

          -  For Group 1, patients who are HER 2 positive.

          -  Any other concurrent uncontrolled illness including, but not limited to, active or
             ongoing symptomatic infection requiring IV antibiotic treatment, uncontrolled
             diabetes, hepatic, renal, or respiratory illness.

          -  Severe or unstable angina, myocardial infarction or ischemia, symptomatic congestive
             heart failure, arterial or venous thromboembolism requiring coronary artery bypass
             graft or stent within the past 6 months or clinically significant cardiac dysrhythmia
             or New York Heart Association class II ~ IV heart disease within 6 months of
             randomization.

          -  Uncontrolled hypertension

          -  Immunocompromised patients, such as patients known to be serologically positive for
             HIV.

          -  Patients with known active Hepatitis B or C infection.

          -  Patients with known active or symptomatic pneumonitis, or history of non-infectious
             pneumonitis requiring steroids.

          -  Diagnosis of a myelodysplastic syndrome/acute myeloid leukemia or its suspicious
             characteristics.

          -  Any unresolved clinically significant Common Terminology Criteria for Adverse Events
             (CTCAE) Grade ≥2 toxicity

          -  Resting ECG with measurable QTcF > 470 msec on 2 or more time points within a 24-hour
             period or family history of long QT syndrome.

          -  Current use of a cytochrome P3A4 inhibitor or inducer and strong uridine diphosphate
             (UDP)-glucuronosyltransferase 1A1 (UGT1A1) inhibitors.