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.
Zoro could react in a rather gruff way. He might say something like 'Nami, are you out of your mind?' But deep down, he might be secretly flattered. After all, Nami is an important person in his life. He'll probably need some time to process this unexpected turn of events and think about what it means for their relationship, both as nakama and potentially something more.
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.
Luffy would be over the moon. He loves new adventures and a new baby on the ship would be like a new kind of adventure for him. He'd probably want to play with the baby all the time. Sanji would be a bit jealous at first because he has a crush on Nami, but then he'd also be really happy for them and cook all kinds of delicious food for the new mother and baby. Usopp would be making up all kinds of tall tales about the baby's future, like how the baby will be the greatest pirate ever.
Killua is likely to be completely devastated. He might go into a state of shock at first, not believing that Gon is really gone. Then he could become very angry, either at himself for not being able to protect Gon or at the situation that led to Gon's death.
The crew's reaction would be mixed with shock and joy. Franky would start building baby - friendly gadgets and cribs for them. Brook would be playing soothing music for Robin and Zoro to keep them relaxed. At first, some of the crew might be worried about how this would affect their adventures. But overall, they would come together. For example, Zoro's tough - guy image might change a bit, and the crew would tease him good - naturedly. Robin, being the more reserved one, might be a bit embarrassed by all the attention at first, but she would appreciate the love and care from the crew.