• Histologically confirmed locally advanced, recurrent or metastatic non resectable adenocarcinoma of the stomach, oesophagus or gastroesophageal junction (GEJ) ineligible to curative treatment.

• No dysphagia or difficulty in swallowing.

• No overexpression/amplification of HER2 (IHC 0 or 1+; if IHC is 2+, HIS must be negative). Known combined positive scor (CPS) PD-L1 score (result in % with the name of the method used). The microsatellite and mismatch repair (MMR) status of patient's tumour (MSI/MSS and pMMR/dMMR) must also be known at the time of screening (IHC and PCR tests have to be done).

• At least one evaluable lesion according to RECIST v1.1 outside any previously irradiated area.

• No prior palliative chemotherapy.

• Age ≥18 years old.

• Patient eligible for FOLFOX chemotherapy

• Adequate organs function:

‣ Absolute neutrophils count ≥1.5x10⁹/L

⁃ Platelets count ≥100x10⁹/L

⁃ Haemoglobin ≥9 g/L

⁃ Serum bilirubin levels \<2 times upper limit of normal (ULN), up to 2.5 times ULN in case of hepatic metastasis (biliary drainage allowed)

⁃ Transaminases \<5 times ULN

⁃ Creatinine clearance \>40 mL/min

• No Dihydropyrimidine dehydrogenase (DPD) deficiency (uracilemia \<16 ng/ml)

⁃ Women of childbearing potential must have a negative serum or urine pregnancy test done within 14 days before the first study treatment.

⁃ Patients must agree to use adequate contraception methods for the duration of study treatment and within 6 months after completing treatment.

⁃ Patients must be affiliated to a Social Security System (or equivalent).

⁃ Patient must have signed and dated a written informed consent form prior to any trial specific procedures. When the patient is physically unable to give their written consent, a trusted person of their choice, independent from the investigator or the sponsor, can confirm in writing the patient's consent.

⁃ Availability of archived tumour material for ancillary studies