• Provide signed and dated informed consent and agree to comply with all study related activities.

• Male or female patients aged greater than or equal to 18 years.

• Patients must meet the following criteria for each of the respective parts of the study:

• a) Pathological diagnosis of a HER2-positive, unresectable locally advanced or metastatic, gastric / gastroesophageal (GE) junction adenocarcinoma.

• \[Note: For patients who have already undergone HER2 testing, it does not need to be repeated. For patients who have not undergone HER2 testing, the same can be done as part of pre-screening, after taking informed consent\].

• B) Patients must NOT have received any systemic anti-cancer therapy for the treatment of gastric or gastroesophageal (GE) junction adenocarcinoma.

• \[Note: The partial resection of tumor or debulking surgery is allowed but any therapeutic chemotherapy or systemic anti-cancer therapy is not allowed\].

• Eastern Cooperative Oncology Group (ECOG) Performance status of 0 or 1.

• Acceptable bone marrow as described below:

∙ ANC greater than or equal to 1500/μL (without WBC growth factor support).

‣ Platelet count greater than or equal to 100,000/μL (without transfusion support).

‣ Hemoglobin greater than or equal to 9 g/dL (Transfusion is allowed to achieve this Hb).

• Acceptable organ function as described below:

∙ Total Bilirubin less than or equal to 1.5 x ULN (Patients with known Gilbert's syndrome are allowed with a Total Bilirubin less than or equal to 2.5 x ULN).

‣ AST (SGOT) less than or equal to 3 x ULN (less than or equal to 5 × ULN if known liver metastases).

‣ ALT (SGPT) less than or equal to 3 x ULN (less than or equal to 5 × ULN if known liver metastases).

‣ Creatinine clearance (CrCl) greater than or equal to 60 mL/min (either measured or estimated by the Cockcroft-Gault formula). (Cockcroft-Gault formula for estimated creatinine clearance \[eCrCl\]: eCrCl = \[140 - Age\] × Weight \[kg\] × \[0.85 if Female\] / \[72 × serum creatinine (mg/dL)\]).

‣ Albumin greater than or equal to 3.0 g/dL.

• Evidence of measurable disease as per Response Evaluation Criteria in Solid Tumours (RECIST) version 1.1 \[Note: Measurable disease for solid tumors is defined as at least one lesion that can be accurately measured in at least 1 dimension with a minimum size of 10 mm for non-nodal lesions or 15 mm in short axis for nodal lesions\].

• Patients who have not undergone HER2 testing must be willing and able to provide an adequate archived tumor tissue sample or a fresh tumor tissue sample to confirm HER2 status.

• \[Note: If archived sample is not available, and HER2 testing has not been done, then patient must agree to submit fresh tumor sample for an assessment of HER2 status\].

• Left ventricular ejection fraction (LVEF) greater than or equal to 50% by either echocardiogram (ECHO) or multigated acquisition (MUGA) scan at Screening.

⁃ Ability to swallow and retain oral medications.

⁃ Negative serum pregnancy test in women of childbearing potential (WOCBP).

⁃ Women of childbearing potential and men who partner with such a woman of childbearing potential must agree to use one or more of highly effective method(s) for contraception for the duration of the study, i.e., through 28-day follow up visit, after discontinuation of study drug(s).