I think the last page of the manual clearly explain the issue.
As of today, the only airports bought from the MS Marketplace that works with GSX are FSDT ones, because we automatically install the unencrypted version of the airport .BGL in the GSX profiles folder, while installing GSX.
Any other 3rd party airport bought from the MS Marketplace will work or not, depending on the willingness of the original developer to supply users with a similar files, but it's not something we can check ourselves, since there are just too many sceneries there, and if we kept a list, it might get outdated very quickly, and that wouldn't be fair to developers that *did* make these file available without us knowing it, making them left out from the list even if they did the effort to support GSX.
We expect this situation to improve in the next months, because after SU10 will be out officially, there's a new API coming in the SDK, which will allow us to obtain the airport data without having to open the actual scenery file, making the issue of getting access to unencrypted files obsolete. But first SU10 needs to be released, then we need to properly test the new API to be sure it works and has all the data we need, and finally we'll be able to update our software to support it.
