Camrelizumab Plus Pyrotinib Plus Chemotherapy in Human Epidermal Growth Factor Receptor 2 Positive (HER2+) Advanced Gastric or Gastroesophageal Junction (GEJ) Adenocarcinoma

Inclusion Criteria: 1. Age 18 years or older. 2. Histologically or cytologically confirmed diagnosis of locally advanced unresectable or metastatic HER2 positive gastric or GEJ adenocarcinoma. 3. Patients have not received systemic treatment in the past but had disease progression more than 6 months after receiving neoadjuvant therapy or the last of adjuvant therapy could be enrolled or failure of first-line therapy or completion of (new) adjuvant therapy to disease recurrence less than 6 months. 4. HER2-positive defined as either immunohistochemistry (IHC) 3+ or IHC 2+ in combination with fluorescent in-situ hybridization (FISH+ is defined as HER2:CEP17 ratio≥2.0), as assessed by central review on primary or metastatic tumor. 5. ECOG performance status 0-1. 6. At least one measurable lesion exists as defined by RECIST 1.1 . 7. Life expectancy of more than 12 weeks. Exclusion Criteria: 1. Hypersensitivity to Camrelizumab, pyrotinib and study chemotherapy agents and/or to any components. 2. Has received prior therapy with an anti-PD-1, anti-PD-L1, or anti PD L2 agent or with an agent directed to another stimulatory or co-inhibitory T-cell receptor (e.g., cytotoxic T-lymphocyte-associated protein 4 \[CTLA-4\], OX 40, Cluster of Differentiation 137 \[CD137\]). 3. Has an active autoimmune disease that has required systemic treatment in past 2 years. 4. Has a known history of Human Immunodeficiency Virus (HIV) or active hepatitis B and C virus infection. 5. Has had major surgery within 28 days prior to randomization, or anticipation of the need for major surgery during the course of study treatment. 6. Subjects who can not interrupt the using of the drugs that may cause QT prolongation during study. 7. Evidence or history of coagulation disorders such as a grade ≥ 3 (CTC-AE) bleeding event. 8. Known history of psychotropic substance abuse or drug use.