Docetaxel + Oxaliplatin + S-1 in Potentially Operable Gastric or Gastroesophageal Adenocarcinoma

The primary objective of this trial is: * To determine whether it is feasible in locally advanced gastric or gastroesophageal cancer to administer 3 cycles of Docetaxel, Oxaliplatin and S-1 as a chemotherapy scheme and also to determine what toxicities are involved. The secondary objective of this trial are to describe: * The disease free survival at one and two years in that subgroup of patients that has undergone a R0 resection. * The downstaging after 3 cycles of chemotherapy, pCR in that subgroup of patients that have undergone an R0 resection and progression-free survival and overall survival at one and two years.