Does Hisoka know Gon is Ging's son? 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.