When Elie and his father were taken to the camps they ended up in Auschwitz which was one of the worst barracks to inhabit. It was a system of snitching and cruelty that involved the Nazis using prisoners to watch over one another, sometimes exerting unnecessary cruelty over the others in aims to get treated better by the Germans.
Elie's father, Schlomo, simply asked where were the toilets, as he needed to go to the bathroom. The "Gypsy" was by then the prisoner in charge of the supervision of those barracks. As his name entails, the "gypsy" was a man of suspicious background and so he may have been more cruel to the others than the Germans would have been, especially if they were to receive special treatment from the Nazis.
...he slapped my father with such force that he fell down and then crawled back to his place on all fours.
This incident marks yet another sign of the pathetic nature of the situations that the Jews had to undergo at the hands of their torturers.