A Study of Disitamab Vedotin Combined With Trastuzumab and Tislelizumab Versus Chemotherapy Combined With Trastuzumab With or Without Pembrolizumab in HER2-high Expression Advanced Gastric or Gastroesophageal Junction Adenocarcinoma.

Inclusion Criteria:

• Voluntarily consent to participate in the study and sign the informed consent form

• Expected survival period >12 weeks

• ECOG Performance Status 0 or 1

• Histologically confirmed unresectable locally advanced or metastatic --gastric/gastroesophageal junction adenocarcinoma

• No prior systemic therapy for locally advanced or metastatic gastric cancer; ordisease progression or recurrence occurring ≥6 months after completion ofneoadjuvant/adjuvant therapy

• HER2-high expression

• At least one assessable lesion according to RECIST v1.1 criteria

• Adequate organ function

• Female subjects of childbearing potential must have a negative blood pregnancy testwithin 7 days prior to the first treatment, and agree not to breastfeed or donateova from the signing of the informed consent form until 6 months after the lasttreatment. Male subjects must agree not to donate sperm from the signing of theinformed consent form until 6 months after the last treatment.

• Able to understand the study requirements and willing to comply with the study andfollow-up procedures

Exclusion Criteria:

• Presence of central nervous system (CNS) metastasis and/or carcinomatous meningitis

• Peripheral neuropathy > Grade 1

• Tumor lesions with a tendency to bleed

• Severe gastrointestinal dysfunction that may affect drug intake, transport, orabsorption

• Bone metastases with a risk of paraplegia

• Past or current interstitial lung disease, or severely impaired lung function

• Other malignancies within 5 years prior to randomization, except for those expectedto be cured with treatment

• Pregnant or breastfeeding women