First the participant will receive a dose of pembrolizumab alone. Two weeks after this dose, then s/he will receive all 3 study drugs (pembrolizumab, gemcitabine and cisplatin) on the first day of a 3 week "cycle". S/he will then receive 1 or 2 of these drugs on the 8th day of the cycle. The participant will receive the study drugs for 4 cycles for a total of 12 weeks.