The goal of this clinical trial is to evaluate postoperative adaptive adjuvant therapy in patients with gastric or gastroesophageal junction adenocarcinoma after neoadjuvant chemotherapy plus immunotherapy and radical gastrectomy.The main questions it aims to answer are:
1. In patients with poor pathological response, does switching to a alternative postoperative treatment regimen improve survival?
2. In patients with complete pathological response, can observation without routine postoperative treatment maintain favorable survival outcomes? Participants will be assigned to different cohorts according to their pathological response after surgery and will be followed regularly for recurrence, survival, and treatment-related side effects.