I've just got GSX for the first time, I've activated it and I was trying to use it on the Majestic Dash 8. The trucks didn't seem to line up correctly with the doors so I tried playing with the settings to figure it out.
GSX supports the Majestic Dash 8 out of the box, without requiring any special configuration, and nobody reported a problem with the alignment of vehicles. See the attached screenshot.
There are other issues with it, during the pushback, not related to GSX, but with the fact that airplane use a non-standard flight model, and we also added some features in GSX that Majestic could use to detect if GSX is pushing, so it's now up to them to fix the remaining issues to have the airplane behave like all others. But this doesn't have anything to do with alignment.
However, when I tried to manually align the trucks with the doors using the 3D editor the view jumped around violently and I couldn't do anything, in fact I had to exit FSX. It's done the same thing every time I've tried going into the editor.
This is usually caused by your joystick null-zone not properly calibrated. Either calibrate the null zone correctly, or turn off the joystick input (CTRL+K key by default) before entering the editor. However, you shouldn't have any need to edit the airplane to begin with.
Try to remove your custom configuration file, by removing the following folder:
%APPDATA%\Virtuali\Airplane\mjc8q400
Then restart Couatl, so you'll using the configuration that comes with GSX.