Hey Danny,
Instead of zones, you have a combination of categories and scripts.
You define which scripts
gab
will run on (i.e. showthread, search, etc...). Each script then runs 1 or more billboards. A billboard can have 1 or more ads (though they have to be the same type) and they can be limited to a particular category.
Its a bit complex, but what you are trying to do is easy to achieve by:
Creating 2 billboards (vert and horizontal). Limit the ads to be pulled from corresponding categories (i.e. cateogries that hold only vertical ads or horizontal ads).
Then assign those billboards accordingly in your script.
Hope that didnt muddle it up too much