We use the same method the simulator use to find scenery, and we support:
- Sceneries added with the add-on.xml method, supporting their layering too.
- Sceneries added in the scenery.cfg files, supporting their layering too.
And, just like the sim, the scenery with the highest layer will be used. However, there are some cases where a scenery might not be chosen:
1) The airport must be self-contained. GSX only reads ONE AFCAD at the time. If a scenery only adds some parkings or taxiways but still relies on the default AFCAD, GSX won't use it.
2) The file might be too big. Some freeware authors like to place everything in the airport file, including textures, making the AFCAD unnecessarily large. GSX will automatically skip files which are larger than what a normal AFCAD might be, in order to speed up the cache regeneration file. You can check and control this, see here:
http://www.fsdreamteam.com/forum/index.php/topic,18925.msg131424.html#msg131424