SHR-A1904 Compared With Investigator's Choice of Therapy in Claudin18.2 Positive Patitens With Second-line Advanced or Metastatic Gastric or Gastroesophageal Junction Adenocarcinoma

Inclusion Criteria: 1. Age 18 to 75 years old (including boundary values) 2. Volunteer to participate in this clinical study and sign informed consent; 3. ECOG score 0-1; 4. Expected survival ≥3 months; 5. Gastric or Gastroesophageal Junction Adenocarcinoma; 6. positive CLDN18.2 expression in tumor tissue; 7. There is at least one measurable or evaluable lesion that meets the RECIST 1.1 criteria; 8. Adequate bone marrow and organ function. Exclusion Criteria: 1. Received anti-tumor therapies such as chemotherapy, radiotherapy, biological therapy, targeted therapy, or immunotherapy within 4 weeks before the first dose of the study 2. HER2 posotive (IHC 3+ or IHC 2+/ISH +); 3. Toxicities caused by previous anticancer therapy were not recovered to CTCAE 5.0 Grade≤1; 4. Individuals with Leptomeningeal metastasis or Active brain metastases; 5. Individuals with a history of GI perforation or fistula, unstable GI bleeding; 6. Individuals with a history of severe cardiovascular and cerebrovascular diseases; 7. The researcher determined that there are other situations that are not suitable for participation.