A Study of ASP2138 Together With Chemotherapy and Pembrolizumab in Adults With Gastric Cancer

Inclusion Criteria: * Participant has histologically or cytologically confirmed gastric or GEJ adenocarcinoma. * Participant has radiographically confirmed, locally advanced unresectable or metastatic disease within 28 days prior to randomization. * Participant has predicted life expectancy \>= 12 weeks. * Female participant is not pregnant and at least 1 of the following conditions apply: * Not a women of childbearing potential (WOCBP) * WOCBP who has a negative serum pregnancy test at screening and agrees to follow the contraceptive guidance from the time of informed consent through at least 9 months after the final oxaliplatin administration and 6 months after the final administration of all other study intervention. (Note: For screening, participants with elevated serum human chorionic gonadotropin (HCG) and a demonstrated nonpregnant status through additional testing are eligible.) * Female participant must not be breastfeeding or lactating starting at screening and throughout the investigational period, and at least 9 months after the final oxaliplatin administration and 6 months after the final administration of all other study intervention. * Female participant must not donate ova starting at first administration of study intervention and throughout the investigational period, and for 9 months after the final oxaliplatin administration and 6 months after the final administration of all other study intervention. * Male participant must agree to use contraception with female partner(s) of childbearing potential (including breastfeeding partner) throughout the treatment period and for 6 months after the final administration of study intervention. * Male participant must agree to remain abstinent or use a condom with pregnant partner(s) for the duration of the pregnancy throughout the investigational period and for 6 months after the final administration of study intervention. * Male participant must not donate sperm during the treatment period and for 6 months after the final administration of study intervention. * Participant has radiologically evaluable disease (measurable and/or non measurable) according to RECIST V1.1, per investigator assessment, \<= 28 days prior to randomization. For participants 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. * Participant's tumor is HER2-negative (HER2 immunohistochemistry (IHC) score 0+/1+ or HER2 IHC score 2+/in situ hybridization (ISH) negative) as determined by local or central testing. * Participant has provided an formalin-fixed paraffin-embedded (FFPE) tumor sample which meets the requirements of the study as specified in the laboratory manual. * Participant has CLDN18.2-positive tumor as determined by central testing. * Participant has a valid programmed death-ligand 1 (PD-L1) result as determined by central testing of a tumor sample. * Participant with known microsatellite instability-high or mismatch repair deficient status may enroll as long as their tumor expresses PD-L1 combined positive score (CPS) \>= 1 as determined by central IHC testing. * Participant has ECOG performance status 0 to 1. * Participant must meet all of the 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 (transfusion is allowed, but posttransfusion hemoglobin \[24 hours or later following transfusion\] must be \>= 9 g/dL). South Korea Specific: * Participant is \>= 19 years of age at the time of signing informed consent. * Female participant is not pregnant and at least 1 of the following conditions apply: * Not a WOCBP * WOCBP who has a negative serum pregnancy test at screening and agrees to follow the contraceptive guidance from the time of informed consent through at least 15 months after the final oxaliplatin administration and 6 months after the final administration of all other study intervention. (Note: For screening, participants with elevated serum HCG) and a demonstrated nonpregnant status through additional testing are eligible.) * Female participant must not be breastfeeding or lactating starting at screening and throughout the investigational period, and at least 15 months after the final oxaliplatin administration and 6 months after the final administration of all other study intervention. * Female participant must not donate ova starting at first administration of study intervention and throughout the investigational period, and for at least 15 months after the final oxaliplatin administration and 6 months after the final administration of all other study intervention. * Male participant must agree to use contraception with female partner(s) of childbearing potential (including breastfeeding partner) throughout the treatment period, and for 12 months after the final oxaliplatin administration and 6 months after the final administration of all other study intervention. * Male participant must agree to remain abstinent or use a condom with pregnant partner(s) for the duration of the pregnancy throughout the investigational period and for 12 months after the final oxaliplatin administration and 6 months after the final administration of all other study intervention. * Male participant must not donate sperm during the treatment period and for 12 months after the final oxaliplatin administration and 6 months after the final administration of all other study intervention. Japan Specific: * Female participant is not pregnant and at least 1 of the following conditions apply: * Not a WOCBP * WOCBP who has a negative serum pregnancy test at screening with a medical interview and agrees to follow the contraceptive guidance from the time of informed consent through at least 9 months after the final oxalipla