GSX use the standard Simconnect menu window in the most standard way (no hacks) and, more importantly, nothing has ever changed with the last update so, if it doesn't work in full screen mode, something that nobody ever reported before, it's surely something that doesn't have anything to do with GSX, and it's happening entirely inside the sim, since application that use the official way to display text, don't have any control if the sim is running in Windowed mode or Full screen mode, nor they are required to do anything differently.
Try to let the cache update once, while in windowed mode, then exit the sim and use full screen mode on the next session. Normally, GSX won't try to update the cache again, unless a new scenery is installed OR the previous cache regeneration didn't complete, usually due to a corrupted .BGL
Also, it's possible the issue might happen because you have multiple addons trying to write text there. Is the GSX cache regeneration particularly slow ? If yes, you might improve it by disabling the whole FSX folder from antivirus scanning, and add every other folder you might have outside the sim which contains sceneries. That might speed up the cache regeneration A LOT and, possibly, prevent issues if there are too many addons trying to use it, and GSX taking it for too much time, because the cache regeneration is slow, both because you have too many sceneries AND because the antivirus is checking each and every file being opened, that's why you should exclude both the sim and every other scenery location from antivirus scanning.
