I finally figured it out, the key wass assigned to the end key. Just one more question, is the key that switches between the course mode and precision mode also handled by the keyboard itself or it it an fsx key?
On a regular keyboard, is the Numpad 5, which in FSX, by default, controls the "Reset Slew mode" function.
There's a chart in the GSX manual, when the airplane configuration editor, which clearly indicate the default key used, the function assigned in the GSX editor, and the function normally by FSX to that key when the GSX editor is not active.
GSX is not reading the keyboard directly. That would be wrong, because it would prevent you to reassign keys.
If you redefine the "Slew Reset" (which controls the toggle between coarse/precision mode in GSX, ONLY when the editor is active), from the default Numpad 5 to, let's say, a joystick BUTTON, GSX WILL use it, and FSX too so, in that case, the newly assigned joystick button will now control the Reset function when you are in the default FSX Slew mode OR the toggle between coarse/precision mode in GSX, when GSX is inside the airplane configuration editor.
Basically, GSX doesn't care about the keyboard directly, it only intercepts an FSX EVENT, regardless how you configured it.