This phase III trial compares the effect of modified fluorouracil, leucovorin calcium, oxaliplatin, and irinotecan (mFOLFIRINOX) to modified fluorouracil, leucovorin calcium, and oxaliplatin (mFOLFOX) for the treatment of advanced, unresectable, or metastatic HER2 negative esophageal, gastroesophageal junction, and gastric adenocarcinoma.