XELOX Combined With Sintilimab and HBO for Advanced or Metastatic GC/GEJC

Inclusion criteria: 1.Diagnosis: Histologically or cytologically confirmed adenocarcinoma of the stomach or gastroesophageal junction (including signet ring cell carcinoma, mucinous adenocarcinoma, and hepatoid adenocarcinoma). 2.Disease status: The presence of metastatic disease, attributable to either recurrence or distant dissemination, was established through a combination of radiologic or surgical assessments. 3.Survival Expectancy: Predicted to live more than 3 months. 4.Age: 18-75 years. 5.Prior treatments: 1. There are no previous antitumor treatments (chemotherapy, radiotherapy, targeted therapy, immunotherapy, interventional therapy, etc.). 2. If patients previously received adjuvant or neoadjuvant therapy, the last treatment must have been completed at least 6 months before randomization, with no recurrence or disease progression during treatment. 3. Palliative radiotherapy is allowed if it is completed at least 2 weeks before the first study treatment. 4. The use of prior anti-tumor traditional Chinese medicine is allowed if it is discontinued at least 2 weeks before randomization. 6.Performance Status: ECOG PS ≤1. 7.Assessable lesion: At least one measurable lesion per the RECIST 1.1 criteria. 8.Pathological samples: Patients whose archived or fresh pathological tissue was obtained within 6 months before signing informed consent, which was sufficient for PD-L1 testing with obtainable results, were included. 9.Organ function: 1. Hematology (no transfusion or G-CSF use within 14 days before screening): Hemoglobin ≥90 g/L. Absolute neutrophil count (ANC) ≥1.5×10⁹/L. Platelet count ≥75×10⁹/L. 2. Biochemistry (no albumin use within 14 days before screening): Albumin ≥28 g/L. Total bilirubin ≤1.5×ULN. AST and ALT levels were ≤3×ULN (≤5×ULN if liver metastasis was present). creatinine ≤1.5×ULN. (3)Coagulation: INR or PT ≤1.5×ULN. APTT ≤1.5×ULN. 10.Systemic treatment history: No systemic treatment (including adjuvant/neoadjuvant) was given within the past 6 months after sample collection for randomization. 11.Toxicity: Prior antitumor treatment or surgery-related acute toxic reactions were resolved to grade 0-1 per NCI CTCAE v5.0 or to levels specified by the inclusion/exclusion criteria. 12.Contraception: Strict contraception measures. 13.Consent and Compliance: Signed informed consent, willing and able to comply with study visits, treatments, lab tests, and procedures. Exclusion criteria: 1. HER2 status: HER2-positive (HER2 3+ or 2+ \& FISH+). 2. Tumor type: Nonadenocarcinoma gastric cancers, including squamous cell carcinoma, undifferentiated carcinoma, or mixed histological types. 3. CNS Metastasis: Uncontrolled or symptomatic active CNS metastasis (e.g., clinical symptoms, brain edema, spinal cord compression, carcinomatous meningitis, soft meningeal disease, or progressive growth). 4. Fluid accumulation: Uncontrolled pleural effusion or ascites treated with drainage within 14 days before randomization; symptomatic or moderate to large pericardial effusion. 5. Weight loss: Weight loss \>20% within 2 months before randomization. 6. Recent treatments: (1)Major surgery within 28 days before randomization (diagnostic biopsies and PICC placement allowed). (2)Immunosuppressive drugs should be used within 7 days before randomization, excluding nasal/inhaled corticosteroids or physiological-dose systemic steroids (≤10 mg/day prednisone or equivalent). (3)Live attenuated vaccines were administered within 28 days before randomization, during the study, or within 60 days after treatment ended. (4)Antitumor treatments were administered within 28 days before randomization (chemotherapy, radiotherapy, immunotherapy, endocrine therapy, targeted therapy, biological therapy, or tumor embolization). 7.Other Malignancies: Patients were diagnosed with any other malignancy within 3 years before study entry, except for localized and cured basal cell carcinoma, squamous or superficial bladder carcinoma, cervical carcinoma in situ, ductal carcinoma in situ of the breast, and papillary thyroid carcinoma. 8.Autoimmune Diseases: Any active, known, or suspected autoimmune disease. Stable conditions not requiring systemic immunosuppression are allowed (e.g., type I diabetes; hypothyroid diabetes managed with hormone replacement; and skin diseases not needing systemic treatment, such as vitiligo, psoriasis, and alopecia). 9.Neurological/Psychiatric Conditions: Uncontrolled epilepsy, congenital spherocytosis, claustrophobia, or angle-closure glaucoma. 10.Immune therapy history: Prior treatment with anti-PD-1/PD-L1 antibodies, anti-CTLA-4 antibodies, or other T-cell costimulation/checkpoint pathway drugs. 11.Bleeding/thrombosis events: significant bleeding symptoms or tendencies within 3 months before randomization; gastrointestinal perforation or fistula within 6 months; thrombotic events (e.g., stroke, deep vein thrombosis, pulmonary embolism) within 6 months. 12.Major Vascular Disease: Major vascular disease within 6 months before study treatment (e.g., aortic aneurysm needing surgery or recent peripheral arterial thrombosis). 13.Wounds and fractures: Severe, unhealed, or open wounds; active ulcers; or untreated fractures. 14.Neuropathy: Peripheral neuropathy \>Grade 1. 15.Intestinal Obstruction: History of intestinal obstruction or related symptoms within 6 months before study treatment. Patients treated surgically to resolve incomplete obstructions at initial diagnosis may be included. 16.Systemic diseases: Interstitial lung disease, noninfectious inflammation, or uncontrolled systemic diseases (e.g., diabetes, hypertension, pulmonary fibrosis, acute pneumonia). 17.Drug Allergies: Patients with known severe allergic reactions to the study drugs or any monoclonal antibodies. 18.HIV/AIDS: HIV infection or known AIDS. 19.Hepatitis: Untreated active hepatitis B (HBV-DNA ≥500 IU/ml), hepatitis C (HCV-RNA above the detection limit), or coinfection with HBV and HCV. 20.Cardiac c