Hello,
A short feedback from my side:
I use nearly all FSDT Airports in P3Dv5, so Hawaiian Airports Vol. 1 and 2, KFLL, KMEM, CYVR etc. In addition, I have some FSDT Airports for MSFS like LSZH, LFSB and CVYR.
After installing GSX Pro, I had the same problem as everybody - COUATL scripting engine not starting in MSFS. I spent the whole afternoon yesterday with troubleshooting. What helped at the end was deinstalling all FSDT Airports, GSX and the Addon Manager folder and only install GSX Pro and the native FSDT Airports for MSFS. As soon as I installed an old FSDT Airport like KFLL, the scripting error was back again. So it seems, at least on my computer, that it is for the time being not possible to run GSX in P3D and GSX Pro in MSFS together. Luckily, my P3D was not messed up by the deinstallation.
I hope that there will be a solution soon because I still use P3D for longhaul flights and I don't want to miss GSX there.
Kind regards,
Daniel