Your mistake is installing GSX *INSIDE* FSX. This is not how our installer defaults to, and it's NOT the best choice. To fix your mistake:
- Uninstall everything from FSDT. Reply YES to the question "Do you want to remove the Addon Manager ?"
- Download the latest installer for GSX. DO NOT use an older installer, one that used to install into the sim by default.
- Reinstall GSX ONLY. Installing the Addon Manager is not required, since it comes with GSX, of course.
- If you have enough space on C:, just accept the default folder. If you don't, select F:, which will install into F:\Addon Manager.