JS107 vs Investigator's Choice as Second-line or Later Therapy for Advanced CLDN18.2-Positive Gastricor GEJ Adenocarcinoma.

Inclusion Criteria: 1. Voluntarily participate in this study, have ICF signed after sufficient informed consent, and have good compliance. 2. Age ≥ 18 years, male or female. 3. ECOG PS 0 or 1. 4. Expected survival period≥ 3 months. 5. Patients with HER2 negative G/GEJ adenocarcinoma confirmed by histology/cytology. 6. Patients who have received at least one prior line of systemic treatments and developed PD, and the previous treatment must include fluorouracil and platinum. 7. Fresh or archival tumor tissue (blocks of formalin-fixed, paraffin-embedded \[FFPE\] tissue or unstained FFPE tumor tissue sections) must be available and comfirmed CLDN18.2 positivity by for central laboratory through immunohistochemistry (IHC) before randomization. 8. Having ≥ 1 measurable lesion according to RECIST v1.1 (per investigator assessment). 9. Any AEs and/or complications caused by previous therapies including surgery or radiotherapy have been adequately resolved to Grade 0 or 1 (per NCI-CTCAE v5.0 criteria) or have been stabilized in the judgment of investigators. Exclusion Criteria: 1. Previous treatment with any drug or cellular therapy targeting CLDN18.2 (except CLDN18.2 monotarget monoclonal antibody). 2. Previously treated with an ADC loaded with a tubulin inhibitor. 3. Received strong CYP3A inhibitor or inducer within 2 weeks or 5 half-lives prior to randomization, whichever is longer. 4. Use of chemotherapy, immunotherapy or other anti-tumor therapies or participation in other clinical trials within 3 weeks prior to randomization, or use of oral fluorouracil, small molecule targeted drugs or traditional Chinese medicine for gastric cancer within 2 weeks prior to randomization. 5. Major surgery (requiring general anaesthesia and \>24 hours of Hospitalisation) or other clincal trial drug treatment within 4 weeks prior to randomization, or radiotherapy within 2 weeks prior to randomization. 6. Imaging demonstrating brain metastases (except patients who have completed whole brain radiotherapy or local therapy (such as surgery), have discontinued prednisone for at least 4 weeks prior to randomization, and have stable radiologically confirmed tumor lesions and no clinical symptoms of tumor during 4 weeks prior to randomization), metastases to meninges, or spinal cord compression. 7. Tumor invades important surrounding structures (e.g., large blood vessels, trachea, etc.) with high risk of rupture and hemorrhage or airway fistula, or metastases to bone with high risk of paraplegia. 8. Thromboembolic events within 3 months prior to randomization (except patients with non-pulmonary Thromboembolism who do not require treatment or have been stably treated with anticoagulants for 14 days or longer prior to randomization). 9. History of other neoplasm malignant within 5 years prior to randomization, except for neoplasm malignant cured after treatment. 10. Having active autoimmune diseases requiring systemic treatment (i.e., immunologic modulator, corticosteroid, or Immunosuppression) within 2 years prior to randomization; replacement therapy (such as thyroid hormone, Insulin, or physiologic corticosteroid replacement therapy due to adrenal or pituitary insufficiency) is not considered systemic treatment. 11. Known severe allergic reaction to any component in the investigational drug formula.