Inclusion Criteria:
1. Age 18-70 years (inclusive) at the time of informed consent; sex unrestricted.
2. Voluntary participation: the subject (or legally authorized representative) must provide written informed consent (ICF, Informed Consent Form).
3. Histologically confirmed advanced gastric or gastroesophageal junction adenocarcinoma that has progressed after ≥ 2 prior systemic regimens, OR histologically confirmed advanced pancreatic adenocarcinoma that has progressed after ≥ 1 prior systemic regimen.
4. CLDN18.2 positivity in archival or fresh tumor tissue by immunohistochemistry (IHC): ≥ 40% of tumor cells staining ≥ 2+.
5. At least one measurable lesion per RECIST 1.1 (longest diameter ≥ 10 mm).
6. Estimated life expectancy \> 12 weeks.
7. Eastern Cooperative Oncology Group (ECOG) performance status 0 or 1.
8. Adequate organ function within 14 days prior to leukapheresis, defined by all of the following:
(1) Total bilirubin ≤ 2 × ULN (Upper Limit of Normal); (2) ALT and AST ≤ 2.5 × ULN (≤ 5 × ULN if liver or bone metastases are present); (3) Calculated creatinine clearance (Cockcroft-Gault) ≥ 50 mL/min; (4) Hemoglobin ≥ 90 g/L; (5) White blood cell (WBC) count ≥ 1.5 × 10⁹/L, with documented suitable venous access for leukapheresis and no contraindications to leukapheresis; (6) Absolute neutrophil count (ANC) ≥ 1.5 × 10⁹/L without Granulocyte Colony-Stimulating Factor (G-CSF) or other myeloid growth factors within 7 days of screening labs; (7) Absolute lymphocyte count ≥ 0.5 × 10⁹/L; (8) Platelet count ≥ 80 × 10⁹/L without transfusion within 7 days of screening labs; (9) Prothrombin time (PT) prolongation ≤ 4 s above ULN; (10) Oxygen saturation ≥ 95% on room air. 9: Contraception and pregnancy requirements: Women of childbearing potential (WOCBP) must have a negative serum pregnancy test at screening and again before lymphodepleting chemotherapy; must not be breastfeeding. WOCBP and fertile men must use a highly effective contraceptive method from screening until 12 months after CBG131 infusion or study discontinuation, whichever is later.
Exclusion Criteria:
1. Pregnant or lactating women.
2. Active bacterial or fungal infection within 72 h before lymphodepletion. Subjects receiving prophylactic antibiotics, antifungals or antivirals are allowed if there is no evidence of active infection and the agents are not on the prohibited-medication list.
3. Systemic corticosteroids equivalent to \> 15 mg/day prednisone within 2 weeks before leukapheresis (inhaled or topical steroids permitted). Any systemic glucocorticoids within 7 days before leukapheresis, except inhaled or topical preparations.
4. Live-attenuated vaccine within 4 weeks before leukapheresis or planned during the study.
5. Positive HBsAg or HBcAb with HBV-DNA ≥ ULN; positive HCV antibody with detectable HCV RNA; positive HIV antibody; positive syphilis serology.
6. Prior hypersensitivity to immunotherapy, cyclophosphamide, fludarabine, albumin-bound paclitaxel, tocilizumab, or any component of CBG131 (e.g. DMSO), or other significant allergic history.
7. Anti-cancer therapy within 2 weeks before leukapheresis (or 5 half-lives, whichever is shorter), including surgery, systemic chemotherapy, radiotherapy, interventional procedures, or anti-PD-1/PD-L1 monoclonal antibody (mAb)/Claudin18.2-targeted agents/other investigational drugs within 4 weeks (or 5 half-lives).
8. Prior gene-engineered cellular therapy (e.g. CAR-T, TCR-T, TIL).
9. Major surgery within 4 weeks before leukapheresis or significant trauma, or anticipated major surgery during the study (except cataract or local-anaesthetic procedures).
10. Known or suspected brain metastases.
11. Portal-vein tumor thrombus or tumor thrombus in mesenteric/inferior vena cava on imaging.
12. Central or extensive pulmonary metastases, extensive liver metastases, or widespread bone metastases.
13. History of organ transplantation or on transplant waiting list.
14. Uncontrolled or serious illnesses that could limit participation, including: Diabetes with HbA1c \> 8%, uncontrolled hypertension (\> 160/100 mmHg), Left Ventricular Ejection Fraction (LVEF) \< 50%, congestive heart failure, acute MI, severe arrhythmia, unstable angina within 6 months, pulmonary embolism, COPD, ILD, clinically significant pulmonary-function abnormalities; Active peptic ulcer, active GI bleeding, bleeding diathesis, major GI bleed within 3 months, prior immunotherapy-related bleeding, hypotension requiring vasopressors.
15. Deep ulceration of primary tumor, full-thickness infiltration at anastomotic recurrence, or tumor encasing/invading major vessels on CT/MRI ± endoscopy, carrying high risk of bleeding or perforation.
16. Requirement for warfarin or heparin anticoagulation.
17. Chronic antiplatelet therapy at doses exceeding aspirin 100 mg/day or clopidogrel 75 mg/day.
18. Toxicities from prior therapies not resolved to ≤ Grade 1 or baseline (except alopecia, pigmentation, or laboratory abnormalities allowed by protocol) at informed-consent signature.
19. Clinically significant CNS disease, abnormal neurologic findings, or psychiatric disorder.
20. Other malignancies within 5 years (except adequately treated cervical carcinoma in situ or basal-cell skin cancer).
21. Clinically relevant thyroid dysfunction (TT4, TT3, FT3, FT4, TSH outside normal limits per investigator).
22. Active autoimmune disease (e.g. psoriasis, RA) or any condition requiring chronic immunosuppressive therapy.
23. Known or suspected CNS metastases.
24. Single target lesion \> 4 cm in longest diameter (or \> 4 cm short axis for lymph nodes) before leukapheresis.
25. Symptomatic ascites or ascites requiring repeated paracentesis or intraperitoneal therapy; small-volume radiologic ascites permitted if asymptomatic.
26. Any other condition that, in the opinion of the investigator, renders the subject unsuitable for participation.
