The problem doesn't have anything to do with GSX, it's a known P3D bug, which always respond to all numeric keys from 1 to 0, even if the menu has less entries, and this will cause similar issue with any application with a menu, because the normal behavior should be that, if a menu contains only (for example) 3 entries, the keys from 4 to 0 shouldn't be active.
So, if you have pressed the key, just be sure you don't select a number higher than the maximum number of choices available.
if, instead, the error happens even if you haven't pressed the menu, than it's another issue, and it's caused by another application that has sent a numeric key for any reason and, there's another P3D bug, which is also confirmed, that if a numeric key press is being sent, the NEXT time you'll open a menu, it will get that key press automatically. This, combined with bug #1, will cause the error.
To fix this, be sure that no addon has been assigned the numeric keys from 1 to 0 on the main keyboard.