If the culprit causing this fatal error indeed is the same version of SimConnect that the GSX installer is about to install maybe there's no need throwing a fatal error simply because of that? Instead the installer could just inform you there's no need to install SimConnect since it's already installed...?
If you are seeing the message "there's has been an error running the Simconnt client installer, the result code is:" followed by a number, it doesn't mean you already have Simconnect installed, but it means the Simconnect client installer is either:
1) Missing in your system, because you removed it from the SDK folder inside the sim ( note that this applies ONLY if you are installing under FSX:SE or P3D. We don't launch the Simconnect installer if you install into FSX, because there's no need for it)
OR
2) It's present, but has returned an error code, the "number" indicated in the error message.
In BOTH cases, it's quite correct for the installer to report you about a potential abnormal situation, even if it might not affect GSX (it might or not, but the installer cannot know in advance).
In a NORMAL situation, launching the Simconnect installer when the same version is already installed, should result in the installation skipping away *very* fast, but with no errors.