The only reasons I have had CTDs in P5D so far has been when I have been interacting with GSX. I use Activesky and others, they have not caused problems so far...
Sorry, but that's not the case. The problem is confirmed to be caused by a bug in the standard semi-transparent Simconnect menu window which is also used by the default ATC, and it has been confirmed multiple times ( there's a very long thread on LM forum about this ) by multiple users it can happen even with NO add-ons installed, no GSX and just with the default ATC.
You are probably mislead assuming "it happens only with GSX", only because you are probably using the GSX menu more often. Because, if you followed the thread on LM forum, it's not something as easy to reproduce as "just" calling a menu. Sometimes you must call it many times, sometimes you must fly the sim for a while, but sometimes it happens randomly.
Many users mistakenly assume "it's GSX", but that's just because GSX uses that menu a lot, so the chances for a crash are higher but, again, it's not GSX, it's the sim itself. GSX is only sending a 100% STANDARD Simconnect command to create a menu, which is exactly the same in FSX, P3D1, 2, 3, 4 and 5, nothing has changed in the V5 SDK and the code to call it is EXACTLY the same and we are not required to do any changes to it, because the API is identical.
And of course, the problem has been reported to LM, which are looking into it.