A Study of Anti-PD-1 and LAG-3 Bispecific Antibody(AK129) Combined With Chemotherapy With or Without Cadonilimab in the First-line Treatment of Unresectable Locally Advanced or Metastatic G/ GEJ Adenocarcinoma

Inclusion Criteria: 1. The subject must sign the written informed consent form(ICF) voluntarily. 2. Aged ≥ 18 to ≤ 75 years,male and female at the time of signing the ICF. 3. Histologically confirmed adenocarcinoma of the gastric or gastroesophageal junction (GEJ). 4. Inoperable locally advanced or metastatic gastric or gastroesophageal junction adenocarcinoma. 5. Participants had not previously received systemic therapy for locally advanced or metastatic gastric or gastroesophageal junction adenocarcinoma. 6. According to RECIST v1.1 criteria, subjects had at least one measurable tumor target. Exclusion Criteria: 1. Subjects with known HER2 positive gastric or gastroesophageal junction adenocarcinoma. 2. Histopathological examination confirmed other pathological types. 3. Had received palliative local therapy for non-target lesions within 2 weeks before the first administration. 4. Past treatment with immune checkpoint inhibitors,immune checkpoint agonists,immune cell therapy and any treatment targeting the immune mechanism of tumor action. 5. History of gastrointestinal perforation and fistula within 6 months before the first dose. 6. Active or previously documented inflammatory bowel disease,inability to swallow, malabsorption syndrome. 7. Active malignancy within the last 3 years. 8. Active or untreated brain metastases, meningeal metastases, spinal cord compression, or pia meningeal disease are known to exist. 9. The presence of clinical symptoms of pleural effusion, pericardial effusion, or abdominal effusion, or the need for frequent drainage. 10. There was an active autoimmune disease that required systemic treatment within 2 years prior to the start of the study.