The purpose of this study is to evaluate the clinical efficacy and safety of HLX22 in the HER2+ Locally Adanved or Metastatic Gastric Cancer as the first-line therapy.This study consists of three periods, screening period (28 days), treatment period and follow-up period (including safety follow-up, survival follow-up).Subjects can be enrolled into this study only if they meet inclusion criteria and do not meet exclusion criteria. The enrolled subjects will receive an intravenous infusion of HLX22/placebo and SOC(standard of care: Trastuzumab + XELOX) once every 3 weeks until the loss of clinical benefit, death, intolerable toxicity, withdrawal of informed consent or other reasons as specified in the protocol(whichever occurs earlier).

HLX22(25mg/kg) or HLX22(15mg/kg) or placebo will be administered intravenously [IV] on day 1 of each 3-week cycle. Trastuzumab (8 mg/kg loading dose, 6 mg/kg maintenance dose) will be administered IV on day 1 of each 3-week cycle. SOC chemotherapy is XELOX (1000 mg/m^2 capecitabine administered orally twice daily [BID] on days 1-14 of each 3-week cycle and 130 mg/m^2 oxaliplatin administered IV on Day 1 of each 3-week cycle).

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

Double-blind study