The problem doesn't have anything to do with the GSX installer. At that stage, it's launching the Simconnect installer from your P3D folder so, the message saying ""Another installation is in progress" it's coming from that one. GSX is just waiting for it to end.
First, try after a Windows restart.
the antivirus Kaspersky is closed as it detects some infected temporary files created by GSX installation.
Since it's clear that Kasperky has a bug in this case, maybe the problem is that it's not *really* "closed" or, possibly, it locked the files from the previous Simconnect install (when it HAD blocked them), which might explain why the Simconnect installer is giving you that error.
Again, if you are SURE the antivirus is really disabled, a Windows restart (assuming the antivirus won't reactivate itself on reboot) should fix this.