Inclusion Criteria:
* Histologically confirmed, resectable adenocarcinoma of the stomach, esophagus, or gastroesophageal junction (Stage II or higher, T2N0 with high-risk features).
* Complete surgical resection deemed achievable by multidisciplinary evaluation.
* Willingness to undergo tumor biopsies for biomarker analysis (HER2, FGFR2b, PD-L1, MSI) at screening, progression, or pre/post- surgery.
* Life expectancy ≥ 24 weeks. Eastern Cooperative Oncology Group (ECOG) performance status ≤1.
* Adequate organ function:
* Hematologic: absolute neutrophil count (ANC) ≥1.5 ×10⁹/L, platelets ≥100 ×10⁹/L, hemoglobin ≥ 8 g/dL
* Hepatic: aspartate aminotransferase (AST)/ alanine aminotransferase (ALT) ≤2.5 × ULN, total bilirubin ≤1.5 × upper limit of normal (ULN) (≤2.5 × ULN for Gilbert's)
* Renal: creatinine ≤1.5 × ULN or creatinine clearance ≥50 mL/min/1.73 m²
* Willingness for blood samples to be drawn for research purposes.
* Baseline dihydropyrimidine dehydrogenase (DPD) testing per local guidelines; dosing of 5-FU adjusted for deficiency.
* Use of two effective contraception methods for women of childbearing potential and men during and 4 months after study; pregnant or breastfeeding women excluded.
* Must have the ability to understand and the willingness to sign a written informed consent document.
* Willingness and able to comply with the protocol for the duration of the study, including attending scheduled visits, examinations, the screening procedure, and having their tumor and blood molecularly characterized.
* Understands they must meet all Inclusion and Exclusion criteria in the sub-protocol for the treatment for which they will be later assigned
Additional inclusion criteria for all sub-studies:
• FFPE tumor sample tested at a central laboratory confirming PD-L1 Tumor Area Positivity (TAP) score ≥1%.
Additional inclusion criteria specific to HER2 sub-study:
* Histologically confirmed diagnosis of resectable (i.e., radical surgery eligible), HER2-positive (defined as 3+ HER2 expression by IHC or 2+ HER2 expression by immunohistochemistry (IHC) with in situ hybridization (ISH)-positivity per central assessment) adenocarcinoma of the stomach or esophagus, including the gastroesophageal junction.
* Formalin-fixed, paraffin-embedded (FFPE) tumor sample tested at a central laboratory confirming HER2-positive status.
* Left ventricular ejection fraction (LVEF) ≥50% as determined by either echocardiogram or multiple gated acquisition scan (MUGA).
Exclusion criteria:
* Unresectable disease, peritoneal dissemination, and/or positive cytology on laparoscopy.
* Peripheral neuropathy ≥ Grade 2.
* Active infection.
* Chronic growth factor support for white blood cells or granulocytes.
* Concurrent anti-cancer therapy (exceptions: supportive care medications ≥1 month prior, low molecular weight heparin, prior adjuvant hormonal therapy \>3 years, palliative radiation
≤14 days prior).
* Local/systemic therapy for current gastroesophageal diagnosis (except one FLOX/ mFOLFOX6 dose during screening).
* Significant medical conditions compromising safety or protocol compliance.
* Preexisting cardiac conditions.
* Stroke, transient ischemic attack (TIA), or myocardial infarction within 6 months.
* Prior FGFR-targeted therapy.
* Prior HER2-targeted therapy (except \>5 years prior for breast cancer).
* Prior checkpoint inhibitor therapy (anti-PD-1/PD- L1/PD-L2).
* Conditions preventing safe surgery/biopsy.
* Malabsorption syndrome or inability to swallow oral medication.
* Pregnant or nursing women
* History of stem cell or organ transplant.
* Major surgery within 14 days.
* Other malignancy within 3 years (exceptions: basal/squamous cell carcinoma, in situ malignancy, low-risk prostate cancer).
* Systemic corticosteroids \>10 mg prednisone/ day or immunosuppressive therapy ≤14 days prior (exceptions: adrenal replacement, topical/ inhaled, short prophylactic courses).
* HIV with CD4 \<350, active hepatitis B or C.
* Active or relapsing autoimmune disease (exceptions: controlled type I diabetes, hypothyroidism on replacement, controlled celiac disease, mild skin disease, other non- recurrent diseases).
Additional exclusion criteria for all sub-studies:
* History of hypersensitivity or contraindications to any active substance/active ingredient of any study medication, including chemotherapy components monoclonal antibodies, recombinant proteins, and/or any of the excipients listed in the ingredients of any drug formulation
* Poorly controlled seizures
* Clinically significant bleeding (CTCAE ≥ Grade 3) from the gastrointestinal (GI) tract within 4 weeks prior to enrollment.
* Administered a live vaccine ≤ 4 weeks prior to enrollment
* History of interstitial lung disease or non-infectious pneumonitis, or with severe dyspnea at rest or requiring supplementary oxygen therapy.
* Treated with another investigational product within 28 days of enrolment.
Exclusion criteria specific to HER2 sub-study:
* Total lifetime anthracycline load exceeding 360 mg/m² of doxorubicin or equivalent
* QTc Fridericia (QTcF) \>470 ms. Note: For subjects with longer QTcF on initial electrocardiogram (ECG), follow-up ECG may be performed in triplicate to determine eligibility (e.g., after correction of electrolyte abnormalities, or discontinuation of QT-prolonging drugs).
* Ongoing Grade 2 or greater diarrhea
