GSX doesn't know anything about your keyboard. As long the keyboard works correctly in Windows, there's no difference whatsoever between different keyboard models.
If you cannot assign keys to your liking, it means they are used by something else, either FSX itself, or another FSX add-on, or even another software that is running in your system.
You simply must find free keys, that's it.
Usually, key combinations with TAB are free. For example, TAB+F12. Or, if some rougue addon is stealing the whole F12 key with no regard to modifiers, you can try something else like TAB+F