I'm very sorry guys, but I found the problem and, this time it was MY FAULT.
I was trying to Debug the issues using the Debug version of GSX, so I could manually set the variables, and that worked fine, so I couldn't explain why it happened to you so, after hours and hours of debug, and checking, and going through every line of code in the Addon Manager, Couatl, GSX, step by step, I finally found the cause:
I must inadvertently removed ONE LINE of code in the RELEASE (only) version of GSX and this one was exactly the one that resets all GSX variables to 0 when GSX starts!
The line was still there in the Debug version, which is what I had to use to troubleshoot the problem, and that was what mislead me thinking it was all fine, because it was, in Debug mode...
Without that initial reset, all those variables were unknown to the sim, until something else wrote to them, which could have been either the airplane code, but also my tests made using the command line, which further mislead me it was working correctly because, while testing, I "woke up" the variables which were otherwise not initialized.
The fix is of course already online now in the FSDT Live Updater, and I apologize again for all the troubles caused. I'm sure this will fix any similar issue of problems with GSX airplane integration.