Gastric Junction Adenocarcinoma

The goal of this clinical trial is to learn if sorbitol works to enhance the therapeutic effect of neoadjuvant chemotherapy combined with Tirellizumab (PD-1 inhibitor) in patients with locally advanced gastric cancer. It will also learn about the safety of sorbitol. The main questions it aims to answer are: Does sorbitol enhance the therapeutic effect of immunotherapy and increase the major response rate in patients with locally advanced gastric cancer? Does sorbitol with neoadjuvant chemotherapy combined with Tirellizumab (PD-1 inhibitor) can improve the prognosis of patients with locally advanced gastric cancer? Researchers will compare sorbitol to a placebo (a look-alike substance that contains no drug) to see if sorbitol works to enhance the therapeutic effect of neoadjuvant chemotherapy combined with Tirellizumab (PD-1 inhibitor) in patients with locally advanced gastric cancer. . Participants will: Take sorbitol or a placebo every day for 3 months in 3 treatment cycles Visit the clinic once every 4 weeks for checkups and tests Keep a diary of their symptoms and the number of times they use a rescue inhaler Participants will follow up as planned until PD occurs, informed consent is withdrawn, or follow-up is lost (whichever occurs first). After the end of treatment and safety follow-up, all subjects will be followed up for survival (OS data collected every 3 months ±14 days)..