IMHO it would be easier to make changes without having first to load P3D.
But since GSX is a P3D *plugin*, which cannot work stand-alone, what you are asking is not possible. Of course, you can edit the settings file directly, by opening the following file with Notepad:
%APPDATA%\Virtuali\Couatladdons.ini
All settings related to GSX are in the [GSX] section. Just be very careful because, if you do a mistake and put wrong/unexpected values, it's possible GSX might crash.