This is a phase II, multicenter, open-label study to evaluate the antitumor activity, efficacy and safety of GEN-001 in combination with avelumab as a third line (3L) or greater line treatment which is not received the Standard of Care (SOC) for patients with PD-L1 positive advanced GC/Gastroesophageal Junction Adenocarcinoma who are not received cancer immunotherapy regimens as mono or combination therapy.

Inclusion Criteria:

• Histologically confirmed unresectable, recurrent, locally advanced or metastatic GC/Gastroesophageal Junction Adenocarcinoma
• Eastern Cooperative Oncology Group (ECOG) performance status of 0 or 1
• Have adequate organ functions as defined in the protocol
• Negative childbearing potential
• Have experienced documented objective radiographic or clinical disease progression after 2 or above previous lines of standard therapy.
• PD-L1 positive
• Measurable disease as per RECIST v1.1 defined as at least 1 lesion
• Estimated life expectancy of at least 3 months
• Have ability to swallow and retain oral medication and no clinically significant gastrointestinal abnormalities

Exclusion Criteria:

• Previously received T-cell coregulatory protein inhibitors
• Has clinically significant evidence of ascites by physical exam
• Known prior severe hypersensitivity reactions to monoclonal antibodies or any component in their formulation
• Has active autoimmune disease that has required systemic treatment in the past 2 years
• Current use of immunosuppressive medication
• Have an active infection requiring antibiotics, antifungal or antiviral agents or have received a course of antibiotics within 4 weeks
• Has received a live vaccine within 4 weeks
• Known history or any evidence of active for non-infectious pneumonitis
• Prior solid organ or allogeneic stem cell transplantation
• Has had any investigational or anti-tumor treatment within 4 weeks or 5 half-life periods of starting study treatment, had any major surgeries within 4 weeks
• Has received proton pump inhibitors (PPIs) within 2 weeks
• Has known active central nervous system (CNS) metastases and/or carcinomatous meningitis
• Has clinically significant (i.e., active) cardiovascular disease
• Has other persisting toxicities
• Has other severe acute or chronic medical conditions including immune colitis, inflammatory bowel disease, or psychiatric conditions