I can't offer any suggestions other than it doesn't happen here. I only checked FSDT airports, but your report is not very clear: you indicates only some 3rd party airports, does it mean:
- The marshaller is wrong JUST there
OR
- it's wrong everywhere
OR
- it's wrong JUST in P3D, but ONLY at those airports ?
Have you read the GSX manual about customization ? If you have P3D and FSX on the same system, a customization for a 3rd party scenery should be active for both sims. But if you boot Windows from two different drives when running one sim or the other, if you customized a 3rd party airport in one sim, you must copy the .INI files from one Windows drive to the other, because the %APPDATA%\Virtuali\GSX folder is on a new drive, if Windows boots from there.
If you haven't customized anything, then it's not possible you would see any difference between P3D and FSX, it should be either wrong in both, or correct in both, since the distance calculation is made in the same way and it's based on the cockpit eyepoint. If the eyepoint is the same ( I assume you ARE testing the with the same airplane ), the marshaller should be in the same place in both sim. And of course I've checked it before replying, it IS the same, with the same airplane.
