Do you mean the updating P3D using the Client only ? You can, but it depends how old your P3D Full installation is
If you already have a version that has the new Transparent styled menu, you can update just the client. If, instead, your existing P3D installation still has the old menus (no transparency), then you MUST download and install at least the P3D "Content" installer, otherwise no menu from any 3rd party application (not just GSX) will work.