The problem doesn't have anything to do with GSX, which doesn't try to read or map the keyboard by itself. GSX will only respond to standard events in Slew mode, whatever key is assigned to. Which means:
- If you have reassigned or removed those keys using the standard Controls page in Slew mode, they would either work using the new key, or stop working altogether, if you removed the assignment from the Slew mode page.
F1 is by default assigned to "Altitude Down Fast" in Slew mode
F4 is by default assigned to "Altitude Up Fast" in Slew mode
So, be sure they are correctly assigned or, eventually, use the Reset to default button to set everything as it used to be.
However, even if the key assignments looks fine, it's possible ANOTHER add-on might have taken these keys for itself, and not giving them back to the sim. There's nothing you or we can do to prevent this, you can only disable that add-on while editing.