When you talk about download of the FULL installer for GSX, do you mean the GSX Level 2 Expansion pack only or should the GSX (basic) be installed as well (2 installs)?
Since there's only one installer, it's not as if you can go wrong, the download link on both GSX and GSX L2 pages points to exactly the same file so, even if you don't understand which one is which, it's impossible to get the "wrong" one by mistake, since they both points to the same file.
IMPORTANT - This is not a Stand-Alone product! It's an expansion for GSX, which won't work unless GSX is already active
It means you must BUY a Separate License for GSX L2, and it won't work if you don't have a GSX base license already. But they are coming together, in the same installer (the one and only GSX Full installer we have), so they will separately work in Trial or Full version, depending on the status of their individual Licenses.
Well, in fact, there are *two* installers now, one for FSX and one for P3D4, but I guess that should be abundantly clear.