You are not supposed to manage an FSDT scenery that has been bought on the MS Marketplace with the FSDT Installer.
When the installer updates GSX Pro or does an Update all, if a version of an FSDT scenery is found in the Official folder, an unencrypted version of the scenery airport .BGL file is being downloaded and placed in the %APPDATA%\Virtuali\GSX\MSFS folder, so GSX can read its data and use it, since it won't be able to read it otherwise, since it's encrypted.
This won't really matter if you use the SU10 Navdata API in the GSX Pro Config, which will allow to read encrypted airports with no need to have a separate .BGL for GSX use.
Could you possibly give some extra details about that error ?