Pilot Study of Antiviral Treatment in Combination With Low-dose Gemcitabine in EBV-associated Gastric Cancer (EBVaGC)

Inclusion Criteria: * Patients must be ≥20 years of age. * Advanced gastric adenocarcinoma (including GEJ) that has progressed during or after second-line therapy. * The patient's tumor tissue must have the pre-defined characteristics as follows ; EBV+ * ECOG performance status 0-1 * Patients must have a life expectancy ≥ 4 months from proposed first dose date. * The patient has measureable or evaluable disease as determined by standard computed tomography (CT) or magnetic resonance imaging (MRI) imaging. Examples of evaluable, nonmeasurable disease include gastric, peritoneal, or mesenteric thickening in areas of known disease, or peritoneal nodules that are too small to be considered measurable by Response Evaluation Criteria in Solid Tumors (RECIST version 1.1) * Patients must have acceptable bone marrow, liver and renal function measured within 28 days prior to administration of study treatment as defined below: WBC ≥ 3,500 cells/mm3 and ≤ 50,000 cells/mm3, ANC ≥ 1,500 cells/mm3, Hemoglobin ≥ 9 g/dL (transfusion allowed), Platelet count ≥ 100,000 /mm3; Total bilirubin ≤ 1.5 X ULN, AST (SGOT)/ALT (SGPT) ≤ 2.5 x institutional upper limit of normal unless liver metastases are present in which case it must be ≤ 5x ULN; Creatinine clearance ≥60 mL/min * Provision of fully informed consent prior to any study specific procedures. Exclusion Criteria: * The patient has a history severe or unstable heart disease, e.g. coronary artery disease requiring increased dose of anti-anginal medication and/or coronary angioplasty (including stent placement) within 24 months ( congestive heart failure NYHA III or IV, unstable angina, history of myocardial infarction within the last 12 months, clinically significant arrhythmia) * The patient is pregnant or breastfeeding.(For women with pregnancy potential, an effective method of contraception should be agreed.) * The patient has ongoing Uncontrolled systemic diseases (diabetes, hypertension, hypothyroidism, infection, etc.) * The patient has ongoing central nervous system malignancy. (except for lesions that have been completely removed or underwent anterior brain radiotherapy/gamma knife procedure) * The patient has a history of allergic reactions to gemcitabine, Valganciclovir, aciclovir, valacyclovir ganciclovir * Patients with interstitial pneumonia or pulmonary fibrosis with clinical symptoms