It sounded like if Illumi was to kill Gon, Hisoka would just regrettably relent. Hisoka wants strong people to fight, and that's the only reason he's "raising" Gon. There are always strong people around to fight however, so Gon isn't terribly unique in that sense.
Killua is a brother to Illumi and Illumi is some sort of special ally/friend of Hisoka's though, so those two relationships would both take priority over Gon/Hisoka.