I. To determine the safety and tolerability of pembrolizumab when combined with radiotherapy plus carboplatin and paclitaxel in locally advanced gastroesophageal junction (GEJ)/cardia adenocarcinoma. (Phase Ib) II. To determine the safety and tolerability of pembrolizumab when combined with oxaliplatin, leucovorin calcium, and fluorouracil (mFOLFOX6) (without radiation) in locally advanced GEJ/cardia adenocarcinoma. (Phase Ib) (re-initiated) III. To evaluate the pathological complete response (pathCR) rate of pembrolizumab when combined with radiotherapy plus carboplatin and paclitaxel in locally advanced GEJ/cardia adenocarcinoma. (Phase II) IV. To determine the 1-year progression-free survival (PFS) rate of pembrolizumab when combined with mFOLFOX6 (without radiation) in locally advanced GEJ/cardia adenocarcinoma. (Phase II) (only if Phase Ib is re-initiated)