Efficacy and Safety of Neoadjuvant Nivolumab Plus SOX Versus Nivolumab Plus FLOT in Patients With HER2-negative Gastric and Gastroesophageal Junction Adenocarcinoma

The goal of the study is to learn about Safety and efficacy of preoperative adjuvant SOX regimen combined with nivolumab versus FLOT Regimen with nivolumab in HER2-negative Gastric or Gastroesophageal Junction Adenocarcinoma. The main question it aims to answer are: * Safety and efficacy of preoperative adjuvant SOX regimen combined with nivolumab versus FLOT regimen with nivolumab for the treatment of HER2-negative Gastric or Gastroesophageal Junction Adenocarcinoma. * Disease-free survival of preoperative adjuvant SOX plus nivolumab and FLOT plus nivolumab for HER2-negative Gastric or Gastroesophageal Junction Adenocarcinoma. Participants will be divided into two groups to use a FLOT chemotherapy regimen plus nivolumab (one group) and a SOX chemotherapy regimen plus nivolumab (another group). Researchers would compare tumor regression grade, adverse effects and survival benefit of two neoadjuvant regimens.