A Study of ASP2138 Given by Itself or Given With Other Cancer Treatments in Adults With Stomach Cancer, Gastroesophageal Junction Cancer, or Pancreatic Cancer

Inclusion Criteria (IC): * Participant is considered an adult according to local regulation at the time of signing the informed consent form (ICF). * Female participant is not pregnant, confirmed by serum pregnancy test \&vmedical evaluation by interview \& at least 1 of the following conditions apply: * Not a woman of childbearing potential (WOCBP) * WOCBP who agrees to follow the contraceptive guidance from the time of informed consent through at least 6 months after final study intervention administration. * Female participant must agree not to breastfeed starting at screening \& throughout the study period \& for 6 months after the final study intervention administration. * Female participant must not donate ova starting at screening \& throughout the study period \& for 6 months after the final study intervention administration. * Male participant with female partner(s) of childbearing potential (including breastfeeding partner) must agree to use contraception throughout the treatment period \& for 6 months after the final study intervention administration. * Male participant must not donate sperm during the treatment period \& for 6 months after the final study intervention administration. * Male participant with 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 \& for 6 months after the final study intervention administration. * Participant's tumor sample is positive for claudin (CLDN)18.2 expression by central immunohistochemistry (IHC) testing. * Participant has radiographically-confirmed, locally advanced, unresectable or metastatic disease within 28 days prior to the first dose of study intervention. * Participant has at least 1 measurable disease according to Response Evaluation Criteria in Solid Tumors (RECIST) 1.1 within 28 days prior to the first dose of study intervention. Lesions situated in a previously irradiated area are considered measurable if progression has been demonstrated in such lesions. * Participant has QT interval by Fredericia (QTcF) =\< 470 msec. * Participant agrees not to participate in another interventional study while receiving study Intervention in the present study. * Participant has Eastern Cooperative Oncology Group (ECOG) performance status of 0 or 1. * Participant has predicted life expectancy \>= 12 weeks. * Participant must meet all of criteria based on laboratory tests within 7 days prior to the first dose of study Intervention. In case of multiple laboratory data within this period, the most recent data should be used. If a participant has received a recent blood transfusion, the laboratory tests must be obtained \>= 1 week after any blood transfusion. Monotherapy Disease specific Criteria (MDSC): Gastric/GEJ Cancer * Participant has histologically confirmed metastatic, locally advanced unresectable gastric/gastroesophageal junction (GEJ) adenocarcinoma. * Escalation: Participant with gastric/GEJ adenocarcinoma who has progressed, is intolerant, has refused, or for whom there is no standard approved therapies that impart significant clinical benefit (no limit to the number of prior treatment regimens). * Unique to South Korea: Participant with gastric/GEJ adenocarcinoma who has refused standard approved therapies is not allowed. * Expansion: Participant with gastric/GEJ adenocarcinoma must have received no more than 3 prior lines of systemic chemotherapy treatment. * Unique to EU: Expansion: Participant with gastric/GEJ adenocarcinoma must have received at least first-line standard therapies in the metastatic setting, must have received ramucirumab treatment if eligible \& where ramucirumab is available, \& no more than 3 prior lines of systemic chemotherapy treatment. MDSC: Pancreatic Cancer * Participant has histologically or cytologically confirmed metastatic pancreatic adenocarcinoma. * Escalation: Participant with pancreatic adenocarcinoma who has progressed, is intolerant, has refused, or for whom there is no standard approved therapies that impart significant clinical benefit (no limit to the number of prior treatment regimens). * Unique to South Korea: Participant with pancreatic adenocarcinoma who has refused standard approved therapies is not allowed. * Expansion: Participants with pancreatic adenocarcinoma must have received no more than 2 prior lines of systemic chemotherapy treatment. Note: Participants with locally advanced unresectable pancreatic adenocarcinoma will not be admitted in monotherapy arms. * Unique to EU: Participant with pancreatic adenocarcinoma must have received at least first-line standard therapies in the metastatic setting \& no more than 2 prior lines of systemic chemotherapy treatment. For all participants in combination therapy (CT) administration: * If a participant has received a recent blood transfusion, the laboratory tests must be obtained ≥ 1 week after any blood transfusion. Combination Therapy Disease-specific (CTDS) IC: ASP2138 in Combination with Pembrolizumab \& mFOLFOX6 as First-line Therapy in Gastric/GEJ Cancer * Participant has histologically confirmed diagnosis of gastric/GEJ adenocarcinoma. * Participant has metastatic or locally advanced unresectable gastric/GEJ adenocarcinoma. * Participant with gastric/GEJ adenocarcinoma has progressed \& must not have been previously treated for metastatic disease with either chemotherapy or prior checkpoint inhibitor therapy. * Participant has a human epidermal growth factor receptor 2 (HER2)-negative tumor per local testing. * For CT with oxaliplatin, follow contraception guidelines from time of informed consent through at least 9 months after final study intervention. (Unique to South Korea: For CT with oxaliplatin, follow contraception guidelines from time of informed consent through at least 15 months after final study intervention for women \& 12 months after final study intervention for men