Inclusion Criteria:  Histologically or cytologically confirmed diagnosis of previously untreated, locally advanced unresectable or metastatic HER2 positive gastric adenocarcinoma. HER2-positive defined as either immunohistochemistry (IHC) 3+ or IHC 2+ in combination with in-situ hybridization positive (ISH+) or fluorescent in-situ hybridization (FISH), as assessed by central review on primary or metastatic tumor Has measurable disease as defined by RECIST 1.1 as determined by the IRRC Has a performance status of 0 or 1 on the Eastern Cooperative Oncology Group (ECOG) Performance Scale Has a life expectancy of greater than 6 months Has adequate organ function  Exclusion Criteria:  Has a known additional malignancy that is progressing or has required active treatment within the past 2 years Has history of HER2 targeted therapy Has a history or current evidence of any condition, therapy, or laboratory abnormality that might confound the results of the trial, interfere with the participant's participation for the full duration of the trial, or is not in the best interest of the participant to participate, in the opinion of the treating investigator Is pregnant or breastfeeding, or expecting to conceive or father children within the projected duration of the trial, starting with the screening visit through 7 months after the last dose of trial treatment