Also i wonder why you say GSX Needs VC++ 2005 runtimes, but GSX installs VC++ 2013 runtimes ??
GSX needs the VC+2013 runtimes for ITSELF, because this is in line to LM suggestions to use those runtimes to minimize the chance of conflicts between addons, and it's also the same runtime used by FSX:SE, so it makes sense that we would use that one as well.
However, GSX (like any other addon), needs Simconnect to talk with the sim, and it uses the FSX/SP2/Xpack version, which is why it installs the client when you run under P3D. So, it's the Simconnect CLIENT, which needs the VC++ 2005 SP1 runtime libraries because, even if GSX could theoretically work just with the 2013 ones, without the Simconnect client working correctly, it won't be able to talk with any version of the sim, so you need to install also the libraries needed by the Simconnect client.