Gon would probably be angry at first. He has a strong sense of justice and being kidnapped against his will would surely make him mad. He'd try to fight back immediately if he had the chance.
In some fanfictions, Gon might initially be shocked. He has seen Hisoka as a somewhat eccentric but strong ally. When he discovers Hisoka's more sinister side, like his love for fighting to the death and his disregard for normal moral boundaries in battle, Gon could be conflicted. He might feel a sense of betrayal because he thought Hisoka was more on the side of 'good' in their adventures. However, Gon's own sense of justice and his unwavering belief in his friends could lead him to confront Hisoka about his true nature.
I don't think so. There haven't been any official announcements or indications that the HxH manga will continue. But you never know, maybe there'll be a surprise in the future.