I have this exact issue, I run P3D v3.4.
So it's likely you are running into the very well known problem that has been discussed so many times on the forum, and doesn't have anything to do with GSX, it affects all 3rd party apps with a menu, and it's the result of having updated the P3D Client only, and this will result in lack of files needed to support the Simconnect menus.
This is fixed by:
- Uninstalling the P3D Client
- Downloading and installing the P3D Content
- Reinstalling the P3D Client
This solution is valid assuming you do see the Couatl and GSX menu under the "Addons" menu, which indicates GSX is fully working and you are just missing the menu, because of the above problem.