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; or disease progression or recurrence occurring ≥6 months after completion of neoadjuvant/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 test within 7 days prior to the first treatment, and agree not to breastfeed or donate ova from the signing of the informed consent form until 6 months after the last treatment. Male subjects must agree not to donate sperm from the signing of the informed consent form until 6 months after the last treatment. * Able to understand the study requirements and willing to comply with the study and follow-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, or absorption * 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 expected to be cured with treatment * Pregnant or breastfeeding women