This is a randomized, double-blinded, multicenter study to evaluate the efficacy and safety of HX008 injection combined with irinotecan versus placebo combined with irinotecan as second-line therapy in patients with adcanced gastric or gastroesophageal junction (GEJ) adenocarcinoma who have had tumor progression after first-line treatment with platinum and/or fluropyrimidine therapy.

Inclusion Criteria:

• Understood and signed an informed consent form.
• Age ≥ 18 and ≤ 75 years old, male or female.
• Has histologically- or cytologically-confirmed diagnosis of locally advanced unresectable or metastatic adenocarcinoma of stomach or the esophagogastric junction (GEJ).
• Has experienced documented objective radiographic or clinical disease progression during or after first-line therapy containing platinum and/or fluoropyrimidine therapy.
• Willing to provide tissue for PD-L1 biomarker analysis.
• Has a performance status of 0 or 1 on the Eastern Cooperative Oncology Group (ECOG) Performance Score.
• Life expectancy ≥ 3 months.
• Has adequate organ function.
• Female participants of childbearing potential should have a negative pregnancy within 72 hours before the randomization. Male and female participants should agree to use an adequate method of contraception during the experiment and 1 year after the last administration of the test drugs.

Exclusion Criteria:

• Has squamous cell or undifferentiated gastric cancer.
• Diagnosed additional maliganancy within 3 years prior to randomization with the expection of curatively treated basal cell carcinoma of the skin, squamous cell carcinoma of the skin,curatively resected in situ cervival or non-muscle invasive bladder cancers.
• Has had a prior anti-cancer monoclonal antibody within 4 weeks prior to the first dose of trial treatment or who has not recovered (≤ Grade 1 or at Baseline) from AEs due to agents administered more than 4 weeks earlier.
• Has had prior chemotherapy,radiation therapy or targeted small molecular therapy within 2 weeks prior to the first dose of trial treatment or who has not recovered (≤ Grade 1 or at Baseline) from AEs due to a previously administrated agent.
• Has received prior therapy with an anti-PD-1, or anti-PD-L1, or anti-CTLA-4 agents.
• Has known active central nervous system (CNS) metastases and/or carcinomatous meningitis.
• Has uncontrolled ascites, pleural effusion, or pericardial effusion.
• Has active autoimmune disease that has required systemic treatment in past 2 years.
• Has received a major surgery within 4 weeks prior to randomization.
• Has received system treatment with corticosteroids (dose >10mg/day prednison or other therapeutic hormones) within 2 weeks prior to the first dose of trial treatment.
• Has incomplete intestinal obstruction, active gastrointestinal hemorrhage and perforation.
• Has a history of non-infectious pneumonitis that required steriods or has current pneumonitis.
• Has any serious and/or uncontrolled disease.
• Has active viral infection.
• Has received a live vaccine within 30 days prior to the first dose of trial treatment.
• Has participated in other anticancer drug clinical trials within 4 weeks.
• According to the judgement of the investigators, there are other factors that may lead to the termination of the study.