the boarding gates and parking lots do not coincide in number or position, and when trying to correct them with the airport editor they come out too far or do not appear
There's nothing we can do about these, until we'll release an update to use the Navdata API that will allow reading data from encrypted airports.
How GSX will work ( or not ) in the current version really depends on the relationship between the default airport and the add-on airport. If the default airport has reasonably accurate parking spots placed in the correct positions, GSX might still work on a Marketplace add-on for the same airport. But if the add-on changed a lot, because the default was really inaccurate, it won't really work with GSX, which only see data from the default.