At the top of the page you downloaded the E unit and the GP9 you will find:
QUOTE
Note to F-Unit, E-Unit, and Geep purchasers: The freebies listed at the bottom of the page require either the F-Unit, E-Unit or Geep cabs, depending on the type. If you need to use your existing cab with the bonus downloads (instead of their default based on type), then you'll need to download the Cab Converter file listed in the Cab section below.