Inclusion Criteria:

          -  A female subject is eligible to participate if she is not pregnant (negative serum
             pregnancy test at screening; female subjects with elevated serum beta human chorionic
             gonadotropin (βhCG) and a demonstrated non-pregnant status through additional testing
             are eligible) and at least 1 of the following conditions applies:

               -  Not a woman of childbearing potential (WOCBP)

               -  WOCBP who agrees to follow the contraceptive guidance throughout the treatment
                  period and for 6 months after the final study treatment administration

          -  Female subject must agree not to breastfeed starting at screening and throughout the
             study period, and for 6 months after the final study treatment administration.

          -  Female subject must not donate ova starting at screening and throughout the study
             period, and for 6 months after the final study treatment administration.

          -  A male subject with female partner(s) of childbearing potential:

               -  must agree to use contraception during the treatment period and for 6 months
                  after the final study treatment administration.

          -  A male subject must not donate sperm during the treatment period and for 6 months
             after the final study treatment administration.

          -  Male subject with a pregnant or breastfeeding partner(s) must agree to remain
             abstinent or use a condom for the duration of the pregnancy or time partner is
             breastfeeding throughout the study period and for 6 months after the final study
             treatment administration.

          -  Subject has histologically confirmed diagnosis of Gastric or GEJ adenocarcinoma.

          -  Subject has radiologically confirmed locally advanced unresectable or metastatic
             disease within 28 days prior to randomization.

          -  Subject has radiologically evaluable disease (measurable and/or non-measurable disease
             according to RECIST 1.1), per local assessment, ≤ 28 days prior to randomization. For
             subjects with only 1 evaluable lesion and prior radiotherapy ≤ 3 months before
             randomization, the lesion must either be outside the field of prior radiotherapy or
             have documented progression following radiation therapy.

          -  Subject's tumor expresses CLDN18.2 in ≥ 75% of tumor cells demonstrating moderate to
             strong membranous staining as determined by central IHC testing.

          -  Subject has a HER2-negative tumor as determined by local or central testing on a
             gastric or GEJ tumor specimen.

          -  Subject has ECOG performance status 0 or 1.

          -  Subject has predicted life expectancy ≥ 12 weeks.

          -  Subject must meet all of the following criteria based on the centrally or locally
             analyzed laboratory tests collected within 14 days prior to randomization. In case of
             multiple central laboratory data within this period, the most recent data should be
             used to determine eligibility.

               -  Hemoglobin (Hb) ≥ 9 g/dl. Subjects requiring transfusions are eligible if they
                  have a post-transfusion Hgb ≥ 9 g/dL.

               -  Absolute Neutrophil Count (ANC) ≥ 1.5x10^9/L

               -  Platelets ≥ 100x10^9/L

               -  Albumin ≥ 2.5 g/dL

               -  Total Bilirubin ≤ 1.5 x upper limit of normal (ULN) without liver metastases (or
                  < 3.0 x ULN if liver metastases are present)

               -  Aspartate aminotransferase (AST) and alanine aminotransferase (ALT) ≤ 2.5 x ULN
                  without liver metastases (or ≤ 5 x ULN if liver metastases are present)

               -  Estimated creatinine clearance ≥ 30 mL/min

               -  Prothrombin time/international normalized ratio (PT/INR) and partial
                  thromboplastin time (PTT) ≤ 1.5 x ULN (except for subjects receiving
                  anticoagulation therapy)

        Exclusion Criteria:

          -  Subject has received prior systemic chemotherapy for locally advanced unresectable or
             metastatic gastric or GEJ adenocarcinoma. However, subject may have received either
             neo-adjuvant or adjuvant chemotherapy as long as it was completed at least 6 months
             prior to randomization.

          -  Subject has received radiotherapy for locally advanced unresectable or metastatic
             gastric or GEJ adenocarcinoma ≤ 14 days prior to randomization and has not recovered
             from any related toxicity.

          -  Subject has received treatment with herbal medications or other treatments that have
             known antitumor activity within 28 days prior to randomization.

          -  Subject has received systemic immunosuppressive therapy, including systemic
             corticosteroids within 14 days prior to randomization. Subjects using a physiologic
             replacement dose of hydrocortisone or its equivalent (defined as up to 30 mg per day
             of hydrocortisone or up to 10 mg per day of prednisone), receiving a single dose of
             systemic corticosteroids or receiving systemic corticosteroids as premedication for
             radiologic imaging contrast use are allowed.

          -  Subject has received other investigational agents or devices within 28 days prior to
             randomization.

          -  Subject has prior severe allergic reaction or intolerance to known ingredients of
             zolbetuximab or other monoclonal antibodies, including humanized or chimeric
             antibodies.

          -  Subject has known immediate or delayed hypersensitivity, intolerance or
             contraindication to any component of study treatment.

          -  Subject has prior severe allergic reaction or intolerance to any component of CAPOX.

          -  Subject has known dihydropyrimidine dehydrogenase (DPD) deficiency.

          -  Subject has a complete gastric outlet syndrome or a partial gastric outlet syndrome
             with persistent/recurrent vomiting.

          -  Subject has significant gastric bleeding and/or untreated gastric ulcers that exclude
             the subject from participation.

          -  Subject has a known history of a positive test for human immunodeficiency virus (HIV)
             infection or known active hepatitis B (positive hepatitis B surface antigen (HBs Ag))
             or C infection. NOTE: Screening for these infections should be conducted per local
             requirements.

               -  For subjects who are negative for HBs Ag, but hepatitis B core antibody (HBc Ab)
                  positive, an HB deoxyribonucleic acid (DNA) test will be performed and if
                  positive, the subject will be excluded.

               -  Subjects with positive hepatitis C virus (HCV) serology, but negative HCV
                  ribonucleic acid (RNA) test are eligible.

               -  Subjects treated for HCV with undetectable viral load results are eligible.

          -  Subject has an active autoimmune disease that has required systemic treatment within
             the past 3 months prior to randomization.

          -  Subject has active infection requiring systemic therapy that has not completely
             resolved within 7 days prior to randomization.

          -  Subject has significant cardiovascular disease, including any of the following:

               -  Congestive heart failure (defined as New York Heart Association Class III or IV),
                  myocardial infarction, unstable angina, coronary angioplasty, stenting, coronary
                  artery bypass graft, cerebrovascular accident (CVA) or hypertensive crisis within
                  6 months prior to randomization.

               -  History of clinically significant ventricular arrhythmias (i.e., sustained
                  ventricular tachycardia, ventricular fibrillation or Torsades de Pointes

               -  QTc interval > 450 msec for male subjects; QTc interval > 470 msec for female
                  subjects

               -  History or family history of congenital long QT syndrome

               -  Cardiac arrhythmias requiring anti-arrhythmic medications (Subject with rate
                  controlled atrial fibrillation for > 1 month prior to randomization are
                  eligible).

          -  Subject has a history of central nervous system (CNS) metastases and/or carcinomatous
             meningitis from gastric/GEJ cancer..

          -  Subject has known peripheral sensory neuropathy > grade 1 unless the absence of deep
             tendon reflexes is the sole neurological abnormality.

          -  Subject has had a major surgical procedure ≤ 28 days prior to randomization.

               -  Subject is without complete recovery from a major surgical procedure ≤ 14 days
                  prior to randomization.

          -  Subject has psychiatric illness or social situations that would preclude study
             compliance.

          -  Subject has another malignancy for which treatment is required.

          -  Subject has any concurrent disease, infection, or co-morbid condition that interferes
             with the ability of the subject to participate in the study, which places the subject
             at undue risk or complicates the interpretation of data.