Umberto,
I am baffled and still need some further help, please. The GSX update to which I was referring was 2.3, the one adding new features.
I have uninstalled all my FSdt scenery & GSX
I have downloaded and reinstalled without my firewall being active
I have eliminated the Addon Manager folder from anti virus scans
After this the sceneries were still showing only airbridges, and neither Couatl or GSX were showing in the Addons tab in P3Dv4.2
Next I have removed and reinstalled all my C++ libraries, as you suggested, and repeated the installations as above. All to no avail.
Aside from regular automatic Windows updates, update 2.3 was the only software I have updated on my PC, and I everything was working well beforehand.
Thanks,
Karl