FSDT support obviously exists, which is clearly proven by the number of my posts.
As usual (it shouldn't even required to stress this), you cannot expect or demand the same famous ultra-fast and efficient support FSDT is known for over the weekends.
The problem, of course, doesn't have anything to do with GSX and there's nothing wrong with the GSX installer. The problem is caused by a problem of the installation of the VC++ 2015 runtimes on your system.
The GSX installer, of course, installs the official MS installer for those runtimes, but while starting the official MS installer is usually enough to have the runtimes working on a system that doesn't have them, it cannot FIX a a problem on a system that has a *problem* with those runtimes.
So, the solution in the case is usually installing the runtimes manually, as explained in the thread aptly named "Addon Manager not showing up in P3Dv4 **SOLVED**":
http://www.fsdreamteam.com/forum/index.php/topic,17484.msg122656.html#msg122656