GSX surely won't be able to read .BGL files from the Marketplace, because they are encrypted. However, there might be two possible solutions, and we are preparing to support both:
1) Microsoft/Asobo might add a documented API to access the airport data without requiring to open a .BGL. This would be the better long-term solution, because it will make all these products like GSX or Flight Planners working so much smoother, since they won't have to maintain their own copy of the airport cache and they won't have to scanning the whole Scenery Library at each change. This topic has been suggested on MS/Asobo developers forum, and it's currently one of the most voted suggestions so, we hope it will be added in the future. If this API will ever come, we'll be the first in line to add it to GSX.
2) Assuming MS/Asobo won't add the aforementioned API to access airport data, we already implemented a different way of reading the airport data, which is:
If an airport .BGL is found in the same location of a GSX custom profile ( %APPDATA%\Virtuali\GSX or %APPDATA%\Virtuali\GSX\MSFS for the MSFS version ), GSX will read the .BGL from *there* instead from the Official or Community folder.
We'll use this method to allow OUR OWN airports sold on the Marketplace to work with GSX so, 3rd party developers can just do the same, and if they let you download an unencrypted version of their airport .BGL, ( same as they already let you download a custom GSX .INI profile ), you can place it there, and it won't affect the operation of the scenery in MSFS in any way, it will only used by GSX.