How does Hisoka see Gon exactly? As a rival or as a toy to entertain himself?
Hisoka is a psycho that only seek for strong opponents to fight.

He saw a real potential in Gon since they met (Killua too , Leorio and Kurapika too) and just want him not to die until he becomes a monster worth to confront.

And yes , for psychos , the pleasure they find in their... "activities" are orgasms-like.

Remember just what Hisoka had to do only to "fight" Kuroro , he likes to take his time but in the same time when he fought Gon he could hardly prevent himself from killing him ^^

So we can't say he's a rival , he's just another "toy" for Hisoka , but a special one as he really has the potential to become a monster.