Preview and baking was the job of the Parameter in 0.6, it's the job of the data in 0.7. The new system is far easier to code for (and there's still the possibility to override the drawing on the parameter level if you feel like caching stuff).
I did get it working. A significant number of the GeomGym plug-in users are not able to upgrade to GH0.7, not always their own decision (Lot's of firms don't like to be on the leading edge of new software releases), so my hands are tied.