Why did the Gestapo keep the destination of the deportees a secret from them in Night?
There was no real reason for the German army to tell those being deported where they were being taken. Those being transported were not given any choice of destination and were not given any alternatives as to when they were removed from their residential locations. It was easier to simply round them up and take them away. Rumors could provide answers that served to reassure those left behind that those who had been taken away were not being ill-treated, which bought time for the beginning stages of the Holocaust without public scrutiny or any chance of reprisal or disapproval.
The deportees were quickly forgotten. A few days after they left, it was rumored that they were in Galicia, working, and even that they were content with their fate.
As the war progressed, the Germans also had to open more camps and to transport prisoners from place to place. It was simpler to dispense with informing those being transported of where they were headed.