How do I update GSX for P3D v5+ to version 3.5.8? Mine is stuck on 3.2.0.5765
You are confusing the GSX version number with the Couatl build number, which looks dramatically outdated, since the current version of the couatl engine for P3D is 5.0.0.6013, yours seems from many years ago.
The update is done with the FSDT live update, just be sure your have added the whole Addon Manager folder to the antivirus Exclusion, because it might block the .exe from being updated.