I'm sorry, but you must have done something wrong and, as I've said, there's no reason whatsoever that GSX would work alone, but not after installing PHNL, especially if GSX has been installed last (but this wouldn't be a problem too, if you used a recent installer for PHNL), which is what is happening, according to you last description.
I'll repeat it and confirm it again: the scenery installers downloads the Couatl.exe file from the SAME place as GSX, the two files are exactly the same, it's just not possible that one would work and the other not.
Are you still getting an error from the Couatl.exe now ?