No something that happend on my watch, but ok. From where I stand I saw both an FSX and FSX:SE entry in my registry.
The old FSX one pointed to the old folder now no longer in existence, thanks to a normal uninstall via the "remove software" of FSX boxed.
Whatever the reason, the old registry pointing to the old folder has not uninstalled with it from the registry.
That the installer of GSX visually tells me ONLY FSX:SE is installed, and visually showing me the correct folder of that installation by a mouse-over, but after selecting tells me it will in a no longer existing folder different from the one shown before selecting, seems wrong to me.
Whatever you feel about it: showing one folder to install in, then actually doing it in another, is a bug.
Now I have figured it out and GSX is nicely installed and working correctly. Do what you will with this information, thanks for your assistance.