Nothing in the GSX code opens the menu automatically, it's a feature of the simulator, which restores all panels which were active in the toolbar and reactivate them for the next flight and, since the GSX Toolbar icon requires to be active, otherwise the Hotkey won't work, the pop-up menus won't work, and the text messages won't show ( again, nothing we can control, it's just the way the toolbar menus work, their code is killed the moment you made the icon Inactive ), you need to open it when you first start GSX on a flight.
Which means, if you close the GSX Toolbar icon ( so it goes dark ) when you don't need it anymore, it won't open again when returning to flight.
This is of course explained in the manual, where the menu is discussed:
This is now your normal workflow should be:
• Open the GSX menu from the Toolbar icon when first starting Ground services .
• Don’t close it from the Toolbar while Ground services are performed, always use the Hot-key or the X icon to close the menu.
• Close the menu using the Toolbar icon when you are done with Ground services, usually before take-off.
So, it's enough to close the GSX Toolbar icon by clicking the ICON ( NOT THE X!! ), when you are done using GSX, and it won't be reopened on the next start of the flight.