Found on page 47 of mnual --
Separate .BGL files for GSX use
To overcome this problem, GSX has the ability to load a separate .BGL that is completely
separated from the simulator.
This .BGL should be placed in the same folder used to store GSX custom .INI profiles:
%APPDATA%\Virtuali\GSX\MSFS
Since this folder is outside the normal “Official” or “Community” folders, MSFS
will not see it in any way, and it won’t affect any operation in the sim, which will
continue to use the encrypted version for things like AI operations or ATC.
Only GSX will make use of this file, since the above folder will act as an extra location
with the highest possible priority, and 3rd party developers that sells airports
on the MS Marketplace can simply offer an unencrypted version of the airport
.BGL file to their users, so the scenery can be used by GSX, even if it’s encrypted.