Study Evaluating the Efficacy of the myDIET Software Tool in the Nutritional Management of Patients With Localized and Resectable Gastric or Esogastric Junction Cancer.

Inclusion Criteria: * Patient 18 years of age or older at date of signing informed consent to participate; * Histologically proven diagnosis of gastric adenocarcinoma or OGJ; * Localized, resectable disease; * Patients with an indication for perioperative chemotherapy and/or immunotherapy validated in multidisciplinary consultation meeting, with a scheduled surgery date at least 1 month after enrolment; * Performance status (ECOG): 0-2; * Membership of a social security scheme; * Signed informed consent to participate. Exclusion Criteria: * Other associated solid cancer or haemopathy; * Presence of severe comorbidity (Charlson index \> 9) * Presence of unbalanced dysthyroidism; * History of gastric, duodenal or esophageal surgery; * Inability to comply with study requirements, including : * Impossibility for the patient or his/her caregiver to connect to myDIET; * Difficulty in understanding the written French language; * Psychological incapacity (e.g. excessive vulnerability, psychiatric disorder) or physical incapacity (e.g. physical/motor disability); * Patient under guardianship, curatorship or safeguard of justice; * Patients already participating in a clinical trial or interventional study likely to interfere with the evaluation of the primary endpoint. * Pregnant or breast-feeding patient.