There's nothing hard coded in GSX, which never reads or tries to read the keyboard directly.
What GSX does is to reply to the standard events in Slew mode so, you can surely assign them to anything you like provided, of course, is not already taken by something else. The F7/F8 keys are normally assigned to the "Slew Pitch Down Slow" and "Slew Pitch Down Fast" events, so if they are reassigned in the simulator controls page ( of the Slew mode ), GSX will react to the new assignment.
