GSX only works with an airport, if it can find all the data it needs in a single AFCAD. It cannot take data from multiple AFCADs, like FSX does, so you cannot create just an "enhancement" airport: it must be complete.
"Complete", from a GSX point of view, meaning it must have at least one parking connected to the taxiway layout.
Using ADE, you can create an addon airport based on a default airport, using the "Load Stock Data" option from the Tools menu, which will import all data from that airport from the default scenery, so you can add on that, and obtain a "complete" airport that can be used with GSX.