I can only repeat and confirm that if you see the GSX menu under the "Addons" menu, it means it's working, but it's caused by having updated P3D using the Client only installer, so you must install the P3D Content.
If still not working, either you haven't installed the Content, or you are not seeing the GSX menu under the "Addons" menu. I replied based on you saying you DO have the GSX menu.
Another possible reason, is that you cannot use a parking (usually because the plane is so small) but, if you turned down the "Verbosity" slider in the GSX Settings in order not to be bothered by warnings, so now you won't see the message from GSX telling it cannot work on such parking, which would appear instead of the menu.