It's actually harder to build Zone than it is to build Gengar IMHO.
First Zone's energy lines are far more complex and it likes DCE. I've actually tried it without call. It can go either way.
Second Zone really likes some sort of discard ability which Gengar doesn't really need. You can try Lunatone, Felicity's, and such, but these take up spaces that aren't used in a Gengar deck.
Finally Magneazone has 5 different versions. Personally I like like the SF Lighting one and the DP Free retreat metal one ... this latter let's you get by without the Unown Q and Moonlights that many Gengars run & thus saves you space. The Promo 44 Zone is also really wicked.
Zone also lends itself to some techs like Manectric, Luxray GL, ERL, and such that don't fit in Gengar very well. Still it's hard to fit all that in.