• 1\. Provision of fully informed consent prior to any study specific procedures.

• 2\. Patients must be ≥ 19 years of age

• 3\. Body weight \> 30kg

• 4\. Has a Pathologically documented adenocarcinoma of gastric or gastroesophageal junction with HER2 IHC results.

• 5\. In initial Cohort, HER2 positive (HER2 IHC 3+ or HER2 IHC 2+/ISH positive))

• 6\. Locally advanced unresectable disease by physician's discretion (ex. cT4 or bulky Nx node or localized peritoneal seeding) No evident distant organ metastasis.

• 7\. ECOG performance status PS 0-1 with no deterioration between screening and the first dose of study treatment.

• 8\. Has LVEF ≥ 50% by either echocardiogram (ECHO) or multi-gated acquisition (MUGA) scan within 28 days before treatment

• 9\. Has measurable target disease assessed by the Investigator based on RECIST version 1.1.

• 10\. Has adequate organ and bone marrow, liver and renal function within 14 days before treatment (Note: Transfusion (red blood cell or platelet) or G-CSF administration is not allowed within 14 days prior to the day on which bone marrow function is assessed, or at any time after this day and prior to C1D1.)

‣ Hemoglobin ≥ 9.0 g/dL (≥8.0 g/dL in GC Indications)

⁃ Platelet count ≥100 x 109/L

⁃ Absolute neutrophil count (ANC) ≥ 1.5 x 109/L

⁃ Total bilirubin ≤ 1.5 ULN if no liver metastases \< 3×ULN in the presence of documented Gilbert's syndrome (unconjugated hyperbilirubinemia) or liver metastases at baseline

⁃ AST (SGOT)/ALT (SGPT) ≤ 3.0 x ULN (\< 5×ULN in participants with liver metastases)

⁃ Serum albumin ≥ 2.5 g/dL

⁃ CrCL(Ccr) ≥30mL/min (\> 45ml/min in Rilvegostomig) as determined by Cockcroft Gault (using actual body weight)

⁃ International normalized ratio or Prothrombin time and either partial thromboplastin or activated partial thromboplastin time ≤ 1.5 × ULN

• 11\. Female patients must be using a highly effective method of contraception (refer to the restrictions on P45) during the clinical trial and for 7 months after permanent discontinuation of the study drug. There must be evidence that patients are not breastfeeding, have a negative pregnancy test, or not of childbearing potential by meeting one of the following criteria at screening:

‣ Post-menopausal women defined as aged more than 50 years and amenorrhoeic for at least 12 months following cessation of all exogenous hormonal treatment.

⁃ Documentation of irreversible surgical sterilization by hysterectomy, bilateral oophorectomy, or bilateral salpingectomy, but not tubal ligation.

⁃ Amenorrhoeic for 12 months and serum follicle-stimulating hormone (FSH), luteinizing hormone (LH) and plasma oestradiol levels in the postmenopausal range for the institution.

‣ More detailed information is provided in Appendix G (Definition and accepted contraception for women of childbearing age). Also female participants must not breastfeed and must not donate/retrieve ova from screening to 60days post last dose.

• 12\. Non-sterilized male patients who are sexually active with a female partner of childbearing potential must use a condom with spermicide from screening to 4 months after the final dose of IMP. Complete heterosexual abstinence for the duration of the study and drug washout period is an acceptable contraceptive method if it is in line with the patient's usual lifestyle (consideration must be made to the duration of the clinical trial); however, periodic or occasional abstinence, the rhythm method, and the withdrawal method are not acceptable. It is strongly recommended for the female partners of a male patient to also use at least one highly effective method of contraception throughout this period. In addition, male patients should refrain from fathering a child, or freezing or donating sperm from the time of randomization/enrolment, throughout the study and for 4 months after the last dose of IMP. Preservation of sperm should be considered prior to enrollment in this study.