⁃ Voluntary participation in the clinical study; fully understands and is informed of the study and has signed the Informed Consent Form (ICF).

⁃ The gender is not limited. Age: ≥ 18 years and ≤ 80 years old.

⁃ Gastric or esophagogastric junction adenocarcinoma confirmed by pathology.

⁃ HER2-positive status defined as either IHC score of 3+ or IHC 2+ with amplification proven by fluorescent in situ hybridization (FISH) based on pretreatment endoscopic biopsies.

⁃ Clinical stage at presentation: cT2-T4b, N+/-, M0 as determined by AJCC staging system, 8th edition.

∙ The definition of metastatic lymph nodes: a lymph node must be ≥ 10mm in short axis when assessed by CT scan (CT scan slice thickness recommended to be no greater than 5 mm) according to the guideline of Response Evaluation Criteria in Solid Tumours (RECIST version 1.1)

⁃ Participants with a performance status of 0 \

• 1 on the Eastern Cooperative Oncology Group (ECOG) within 7 days before the first dose of study treatment.

⁃ Life expectancy ≥ 6 months.

⁃ Agreement of providing pretreatment endoscopic biopsies specimens and surgical specimens for biomarker analysis, as well as the peripheral blood, feces and urine sample.

⁃ The functions of the vital organs meet requirements as follow (within 14 days before the first dose of study treatment, participant has not received treatment of recombinant human thrombopoietin or granulocyte stimulating factor):

• Hematological function：

‣ White blood cell count (WBC): 3.5 × 10 \^ 9 / L \

‣ 0 × 10 \^ 9 / L；

⁃ Absolute neutrophil count (ANC) ≥ 1.5 × 10 \^ 9 / L；

⁃ Platelet count (PLT) ≥ 100 × 10 \^ 9 / L；

⁃ Hemoglobin (Hb) ≥ 90 g / L.

∙ Hepatic function：

‣ Total bilirubin (TBIL) ≤ 1.5 × ULN (upper limit of normal);

⁃ Aspartate aminotransferase (AST) ≤ 2.5 × ULN;

⁃ Alanine aminotransferase (ALT) ≤ 2.5 × ULN;

⁃ Albumin (ALB) ≥ 30 g / L.

∙ Renal function：

‣ Creatinine (Cr) ≤ 1.5 × ULN, or creatinine clearance ≥ 60 ml / min for those with creatinine level \> 1.5 × ULN.

∙ Coagulation function：

‣ International normalized ratio (INR) ≤ 1.5;

⁃ Prothrombin time (PT) and activated partial thromboplastin time (APTT) ≤ 1.5 × ULN.

∙ Cardiac function:

‣ The left ventricular ejection fraction (LVEF) value ≥ 55 %, as assessed by echocardiography

⁃ Female of childbearing age must meet requirements: urine or serum pregnancy test must be negative within 7 days before the first dose of study treatment, and she must agree to use adequate contraception methods or keep abstinence (starting with the ICF is signed through 120 days after the last dose of toriplimab, or 210 days after the last dose of trastuzumab, or 180 days after the last dose of chemotherapy, whichever is longer, and should not be breastfeeding. For the male participants must meet requirements: agree to use adequate contraception methods or keep abstinence (starting with the ICF is signed through 120 days after the last dose of toriplimab, or 210 days after the last dose of trastuzumab, or 180 days after the last dose of chemotherapy, whichever is longer).