A black screen on the Create Flight screen can't be possibly related to GSX in any way, since GSX doesn't to anything until you call it with CTRL+F12, and on an airport.
And, it doesn't change your FSX graphic settings, which is the only thing that might be related to a black screen on the Create Flight page.
So no, your problem doesn't have anything to do with GSX, fact you noticed after installing it was probably just a coincidence. Try to reset your video drivers to default settings, or try to reset the FSX.CFG located at %APPDATA%\Microsoft\FSX (which GSX doesn't modify anyway) to its default settings, by moving it to a different folder and let FSX rebuild a new one from scratch.