I wondered if Hisoka knew if Gon is related to Gin. Also if it contributes to his rating of Gon? Do the Zoldycks know who he exactly is?
Yes, in the Hunter Chairman Election arc, it is clear that basically everyone who is a licensed hunter knows that Ging Freecss is Gon's dad, including Hisoka. It is also clear to everyone that Ging and Gon are not emotionally close, per Leorio's outburst in the Hunter Chairman Election. There is no reason to think that the Zoldycks would be in the dark about Ging and Gon being related since they keep tabs on a lot of things, especially where Killua is concerned. However, neither Hisoka nor the Zoldycks express any interest in Ging. Zeno and Silva Zoldyck got involved with Netero in the Chimera Ant arc, but as Ging couldn't be bothered to get involved with it, they have had no reason to have any interaction with or interest in him. Ging is portrayed as liking to play/goof off and being generally disliked by others, so other characters don't have much motivation to care what he does since it is unlikely to affect their agendas. Hisoka has never really evidenced taking personal interest in anyone other than Gon, Killua, Illumi, and Chrollo.
I think Hisoka knew from almost the beginning that he is interested in what Gon wants, and that is his father, so he must have looked him up. Plus, his name is Gon 'Freecss', so it was obvious. Let's not forget to mention that Ging is the best hunter in the world and Hisoka is interested in strong hunters, so, again, he must have looked him up even before meeting Gon.
Also, he's interested in Gon, Killua, Illumi, Chrollo, and Machi as well. Which means, he's not gay and a bae because he wanted to go on a date with Machi. Anyway, in general, he's interested in young people with an amusing behavior or strong people, and he's interested in hugging & cuddling Gon.
Hisoka is interested in very strong and powerful people he want's to kill them, so he's inetrested to Ging Freecss and Isaac Netero, Gon Freecss is he's most interested to, because Hisoka is very strong and powerful that he can see how strong and powerful a person is as seen in the election arc, Hisoka was just amazed in Kuroro Lucifer's power's abilities that's why he want's to kill him, so Kuroro Lucifer was just a killtime to Hisoka, Hisoka was only toying Machi so he has no interest to her, Hisoka was just outsmarting and toying the whole Genei Ryodan group and even this group unite and take on Hisoka they all will die in the hands of Hisoka...