Clinical Study on the Efficacy and Safety of LM-302 Injection Combined With Tislelizumab and Tislelizumab Combined Chemotherapy for the Treatment of Gastric or Gastroesophageal Junction Adenocarcinoma.

Inclusion Criteria: * Willing and able to comply with the study's visit schedule, treatment plan, laboratory tests, and other research procedures. * Age ≥ 18 years. * Histologically or cytologically confirmed adenocarcinoma of the stomach or gastroesophageal junction. * For locally advanced unresectable or metastatic adenocarcinoma of the stomach or gastroesophageal junction that has not previously received any systemic treatment: * Patients who have previously received radical neoadjuvant chemotherapy or adjuvant chemotherapy (radiotherapy) based on a platinum-based combination chemotherapy regimen and have not developed distant metastasis or local recurrence within 6 months after completion of treatment can be included. * The calculation of a 6-month interval is defined as recurrence within the same date range after 6 months. For example, if the end date of the last treatment is January 1, the 6-month period from that date refers to January 1 to July 1 (including July 1). Alternatively, if the end date of the last treatment is August 31, the 6-month period from that date refers to August 31 to February 28 (or February 29 in leap years). * Archived specimens or fresh tumor tissue specimens from ≤3 years ago must be provided for CLDN18.2 and PD-L1 testing (PD-L1 results are not a condition for enrollment). When multiple samples are present, the most recent accessible and qualified sample must be provided. After confirmation by the central laboratory, tumor tissue with positive CLDN18.2 expression is defined as ≥25% tumor cell membrane staining intensity 2+ \& 3+. For patients who have previously received radical neoadjuvant chemotherapy or adjuvant chemotherapy (radiotherapy) based on platinum-containing combination chemotherapy regimens, newly obtained tissue from recurrent or metastatic lesions must be provided for CLDN18.2 and PD-L1 testing. * A negative report on the expression detection of human epidermal growth factor receptor-2 (HER2) in tumor tissue must be provided. The definition of HER2 negative expression is ImmunoHistoChemistry (IHC) score 0/1+, IHC score 2+, and fluorescence in situ hybridization (FISH) negative. * According to the RECIST 1.1 evaluation criteria, there must be at least one measurable lesion. * Expected survival duration ≥ 3 months. * According to the Eastern Cooperative Oncology Group (ECOG) standards, the physical performance status score is 0 or 1. * Good organ function: a. Bone marrow reserve: Platelets (PLT) ≥ 100×10\^9/L, absolute neutrophil count (ANC) ≥ 1.5×10\^9/L, Hemoglobin (Hb) ≥ 90 g/L (no blood transfusion or supportive treatment such as colony-stimulating factors within 14 days prior to hematology parameter examination during the screening period); b. Coagulation function: International normalized ratio (INR) ≤ 1.5, Activated partial thromboplastin time (APTT) ≤ 1.5×upper limit of normal (ULN); c. Liver function: Total bilirubin ≤ 1.5×ULN, Alanine aminotransferase (ALT), Aspartate aminotransferase (AST), Alkaline phosphatase (ALP) ≤ 2.5×ULN (if there is liver metastasis, then ALT and AST ≤ 5×ULN) and Albumin (ALB) ≥ 30 g/L; d. Renal function: Creatinine clearance rate ≥ 50 mL/min (calculated according to the Cockcroft-Gault formula); e. Cardiac function: Left ventricular ejection fraction (LVEF) ≥ 50%; QT interval (QTcF) ≤ 470 ms for females and ≤ 450 ms for males. * Women of childbearing age must have a negative blood pregnancy test within 7 days before administration; any fertile male and female subjects must agree to use highly effective contraception throughout the study medication period and for 7 months after the last dose of the study. Males must agree to refrain from donating sperm during the study period and for 7 months after the last dose of the study. Females must agree to refrain from breastfeeding during the study period. Exclusion Criteria: * Other pathological types confirmed by histopathology, such as squamous cell carcinoma, sarcoma, or undifferentiated carcinoma. * There are gastric cancer lesions confirmed by imaging that are accompanied by cavities or necrosis, and are closely related to major blood vessels, and are assessed by the researchers as posing a high risk of major bleeding. * Patients with known central nervous system (CNS) metastases. Patients with non-meningeal, midbrain, pontine, or spinal cord metastases who are judged by the investigator to have stable brain metastases can be enrolled. Stable brain metastases are defined as patients whose brain metastases have undergone treatment and the condition of the metastases has stabilized (brain imaging examination at least 28 days before randomization shows stable lesions, no neurological symptoms, and no immediate need for local or systemic treatment within 14 days before randomization), with no evidence of new or previously existing brain metastases enlarging. * Clinically uncontrollable third-space effusion, such as moderate or greater volume, requiring long-term catheterization, previous history of intestinal obstruction or paralysis, compartmented ascites, undergoing or planning to undergo local treatment (including drainage, peritoneal shunt, or cell-free concentrated ascites reinfusion therapy) within 14 days prior to screening, or significant increase within 2 weeks after local treatment, meeting any of the above criteria or deemed unsuitable for enrollment by the investigator. * Patients with symptomatic spinal cord compression, or those who are expected to develop symptoms of spinal cord compression if left untreated; or for patients with previously diagnosed and treated spinal cord compression, there is no evidence indicating that the disease was clinically stable for ≥4 weeks before the first study drug administration; except for patients with asymptomatic spinal cord compression indicated by imaging, who are assessed as stable by a specialist and do not require treatment for spinal cord compression at this time. * Accompanied by severe peritonea