Inclusion Criteria:

- Patients with histological or cytological diagnosis of gastric and/or
gastroesophageal junction adenocarcinoma/carcinoma.

- Locally advanced or metastatic inoperable disease.

- Life expectancy ≥12 weeks.

- Patients who may have undergone any type of palliative treatment for localised
disease, including surgical approaches and palliative radiotherapy, but not in the
last four weeks before the trial.

- Adequate bone marrow, hepatic and renal functional reserves (ANC≥1500mm3, PLT≥100mm3,
GFR≥50ml/min by Gault Formula, bilirubin <1.5x, Aspartate Aminotransferase
(AST)/Alanine Aminotransferase (ALT) <2.5x upper normal limit or 5x in the presence
of hepatic metastases).

- Patients must be able to swallow pharmaceutical tablets and to be eligible to receive
intravenous chemotherapy.

- Men or women patients must be at least 18 years old.

- Performance Status Scale 0 or 1 (ECOG).

- Measurable disease according to RECIST 1.1.

- Left ventricular ejection fraction (LVEF) ≥50% (ECHO or MUGA).

- Provision of patient informed consent for participation in the study and for the use
of biological material for research purposes.

- Willingness and ability to comply with scheduled medical visits, therapeutic
treatment programmes, laboratory testing and other study procedures.

Exclusion Criteria:

- Previous systemic first-line therapy.

- Previous therapy with EGFR/HER Tyrosine Kinase Inhibitor (TKI) or other experimental
agent.

- Diagnosis of a second malignancy, except basal cell carcinoma of the squamous
epithelium or in situ carcinoma of any organ, for which an appropriate treatment has
been administered without indications of relapse for 12 months.

- Presence of uncontrolled, active brain metastases (controlled brain metastases are
considered those that have been irradiated and have remained stable for at least 4
weeks after radiation therapy).

- Diagnosis of spinal cord compression or carcinomatous meningitis.

- Any of the following that has occurred within 12 months before the start of the study
treatment: myocardial infarction, serious or unstable angina pectoris,
aortic-coronary or peripheral bypass surgery, symptomatic heart failure, vascular
stroke, or transient ischemic attack, or pulmonary embolism.

- Continuing grade ≥2 heart rate abnormalities; atrial fibrillation of any grade.

- Hypertension uncontrolled by medication treatment (>150/100 mm/Hg despite the
administration of best medical therapy).

- In the case of previous irradiation of locally advanced disease, absence of
measurable tumor sites outside the irradiation field.

- Presence of any other disease which in the opinion of the doctor responsible
constitutes a contraindication for the administration of cisplatin, 5FU or afatinib.

- Diagnosed human immunodeficiency virus (HIV) or disease associated with Acquired
Immunodeficiency Syndrome (AIDS).

- Pregnancy or lactation. Female patients must be surgically sterilised, menopausal, or
must consent to use effective contraception throughout the course of the trial.All
female patients with reproduction ability must undergo a pregnancy test (serum or
urine). The effective contraceptive technique will be determined by the main
investigator or a person authorized by the investigator.

- Any other serious, acute or chronic, medical or psychiatric condition or laboratory
analysis finding which, in the investigator's opinion, could create excessive danger
as regards the patient's participation in the trial or administration of the trial
medication may render a patient ineligible for inclusion in the trial.