What you call the "GSX window", it's not a GSX window.
It's the standard Simconnect in-game menu, which is shared between all 3rd party addons using such menu. It's created by the sim itself, and no addon has any control on its size and position, I guess the size depends on the screen size, but the initial position might always be in the center of the main window, and don't think it can be saved.
This is more a question for LM, and they also made several changes to that window in 3.3 and 3.4.