It means"Serious Installation Error"
It doesn't seem to come from our installer (all installer is in English) and, in fact, if you check your screenshot, it's happening while the installer is installing the Simconnect client, using the official Microsoft installer.
So, your problem doesn't seem to have anything to do with GSX: the official MS Simconnect Installer is having problems installing in your system, likely due to missing system runtimes libraries in your Windows installation.
If you can't even install the official Simconnect client for FSX, GSX (and lots of other addons), won't run so, you should first try to fix this.
Which version of FSX do you use? If it's the Steam version, and you just reinstalled it, you MUST run it at least once before installing any add-on.