Neo-Sequence 1: Neoadjuvant Chemotherapy With or Without Antiangiogenesis and Sequential Immunotherapy for HER2-negative MMR-proficient Locally Advanced Gastric or Gastroesophageal Adenocarcinoma

"Drug: Paclitaxel(albumin-bound) 130mg/m2, ivgtt, D1, Q2w Drug: Oxaliplatin 70 mg/m2, ivgtt, D1, q2w Drug: S-1 40mg (body surface area \< 1.25m2), bid, D1-8, Q2w 50mg (body surface area \>1.25m2, \<1.5 m2), bid, D1-8, Q2w 60mg (body surface area \>1.5m2), bid, D1-8, Q2w Drug: Bevacizumab, 5mg/kg, ivgtt, D1, Q14d Drug: PD-1 antibody, 200mg, ivgtt, D1, Q21d Patients with Her-2 negative, MMR-proficient locally advanced esophagogastric junction or gastric adenocarcinoma will receive 6 cycles of neoadjuvant chemotherapy with or without antiangiogenesis. After comprehensive evaluations, patients who respond to chemotherapy will further receive 2 cycles of PD-1 antibody comibed with antiangiogenesis and chemotherapy as neoadjuvant therapy.~Drug: Pembrolizumab or sintilimab, 100\~200mg, ivgtt, D1, Q3w. Patients will make the final decision of PD -1 antibody according to their economic condition."