• Subjects aged ≥18 and ≤75 years old, male or female.

• ECOG performance status score 0-1.

• Histologically or cytologically confirmed GC or GEJ carcinoma, had been treated with Radical resection (D2, R0 or R1) of gastric cancer.

• Pathological stage:III (8th AJCC TNM).

• Estimated lifetime is greater than 6 months.

• The main organs are functioning well, and the blood test results within 14 days before enrollment should meet the following requirements:

‣ Routine blood test:

• Hemoglobin (HB) ≥90 g/L.

∙ Neutrophil count (ANC) ≥1.5×109/L.

∙ Platelet count (PLT) ≥100×109/L.

⁃ Biochemical test:

• Total bilirubin≤1.5×ULN (upper limit of normal).

∙ Alanine aminotransferase (ALT) and aspartate aminotransferase (AST) ≤ 2.5×ULN; if there is liver metastasis, ALT and AST ≤ 5×ULN.

∙ Serum creatinine (Cr) ≤1.5 ULN or creatinine clearance ≥60mL/min.

• No obvious clinical symptoms of heart disease.

• Must have disease-free status documented by complete physical examination and imaging studies with no evidence of recurrent, residual, or metastatic disease on standard imaging (chest, abdomen, and pelvis captured by CT chest and CT or MRI of abdomen and pelvis) per investigator assessment within 28 days prior to enrollment.

• Females of childbearing potential must have a negative urine or serum pregnancy test within 7 days of randomization and must be willing to use a highly effective method of birth control (Appendix 9) for the duration of the study, and ≥ 120 days after the last dose of penpulimab and 180 days after the last dose of chemotherapy.

• Volunteer to participate in this study and sign an informed consent form.

• Considering that NGS analysis may take up to 10 working days, patients could receive 1 cycle of XELOX chemotherapy after ctDNA sampling.