I believe it has something to do with the dll.xml file being corrupted.
I'm sure that, if your DLL.XML was ALREADY corrupted before installing GSX, you saw a clear message from the GSX installer, offering two choices:
1) reset the DLL.XML to default, which would contain only what you are installing now
OR
2) abort the installation, so you could try to fix your DLL.XML file yourself.
So, it's not that after you installed GSX, your other addons are gone but, instead, after installing GSX and being notified of my DLL.XML to be ALREADY corrupted, I chose to start with a new one from scratch, which is exactly what the GSX installer did.
That's because the installer CANNOT (and should NOT) touch an already-corrupted file, risking destroying entirely so, it can only offer the option of restoring the default one (which doesn't contain any addons), or abort the installation, so you could fix it yourself manually.
Why you DLL.XML was corrupted ? There are only two possible reasons:
- Having edited it by hand previously, and made some kind of mistake
OR
- A defective installer from another product you installed earlier.
The message about your already corrupted DLL.XML file would have told you the reason for the corruption so, if you remember what it said, it might give some clue about how the file was already corrupted before installing GSX, and how to fix it.
Of course, the GSX installed HAS made a backup of your old (corrupted) DLL.XML file, named "DLL.XML_Before_Addon_Manager.xml" in the %APPDATA%\Microsoft\FSX folder. If you copy it over to DLL.XML, you'll be back in the same situation as before installing GSX so, if you run the GSX installer again, you can READ better the message from the installer which says how it's corrupted, and report back.
Alternatively, if you .ZIP the DLL.XML_Before_Addon_Manager.xml and Attach it to a post here (do this, DO NOT post it as is, but ZIP it and Attach it to a post, please), I can tell you why is corrupted, and what might have caused it.