How does Hisoka see Gon exactly? As a rival or as a toy to entertain himself?

Hisoka is a maniac, addicted to pleasure derived from fighting strong opponents and killing them. The pleasure is often depicted in the anime and manga similar to a sexual orgasm. He even makes moaning sounds. Lol. This pleasure seeking is a never ending ladder. The next one must be greater than the previous. He sees Gon as the one who can give him this satisfaction when the right time comes. Killing Gon now would be a huge waste. Gon is his boy toy.

Neither a rival nor a toy. I believe Hisoka has mixed feelings. His stated goal is to allow Gon to "ripen" (achieve his full potential), then defeat him, body and soul. Yet he also has high respect for Gon and will protect him, at least until that "harvest". We even see him as an ally during the Yorknew City and Greed Island arcs.

Being a psycho is complicated. In a very warped way, Hisoka may even love Gon (in addition to a possible sexual attraction). But it is not a love which is likely to end well.

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.