User blog comment:Bennlimos/Understanding the Zoldyck Family's Nen Situation/@comment-4828844-20151018064034/@comment-14902757-20151018064626

Killua told Gon about it. If you want to check, it's Episode 27, @3:46 mark. I don't know how it's been translated in other subs, I haven't read this part of the manga.