Upon reaching the 200th floor of the Heavens Arena, Hisoka reveals that he had followed them there. Hisoka is actually a crowd favorite fighter there, with a perfect record of victories (when he doesn't miss his matches). He prevents Gon and Killua from entering the floor until they have learned Nen, so that they aren't permanently harmed by other floor fighters eager to perform an "initiation" ritual.

Source: Wikipedia