As we announced already yesterday, ALL our installers are entirely compatible with FSX Steam Edition. GSX works perfectly fine in it.
There's a catch, though: If you uninstalled the regular FSX before installing the Steam edition, or you installed the Steam edition on a system that never had FSX installed before, like after a Windows reinstall, you will be missing the Simconnect runtime libraries that are required by almost all FSX add-ons, and unfortunately, they are not installed automatically by the Steam installer.
However, there's a copy of the Simconnect installer in the SDK folder from the Steam installation, here:
steam\steamapps\common\FSX\SDK\Core Utilities Kit\Simconect SDK\Legacy Interfaces\FSX-XPACK\Simconnect.MSI
Launch the installer, and every FSX add-on that required Simconnect (provided you can install it under the Steam edition) will work.