The problem doesn't have anything to do with GSX or any of our software.
It's caused, instead, but ANOTHER 3rd party module you have installed that is using an earlier Simconnect version (the RTM), which is missing the SimConnect_Text API call, introduced by the Simconnect SP1 so, because of this module, the sim is loading the outdated Simconnect.
This has been discussed many times on the forum, and the solution is to ensure that bglmanx.dll is loaded first, which of course is what our installers do, but it seems that it doesn't always work with P3D 3.x, but some users fixed it by copying lines in the dll.xml file under %APPDATA%, to the file into %PROGRAMDATA%, which is always read before anything else. See here for the explanation:
http://www.fsdreamteam.com/forum/index.php/topic,13029.msg97823.html#msg97823Of course, the "real" solution would be other developers would stop using the old Simconnect, because the problem will likely reappear if some of those modules would start to install into %PROGRAMDATA%, but in the meantime, this should work.