Sorry, forgot that electronic fares systems will calculate journeys for airport addons automatically so will show route any permitted, and if they are valid via london would include the "+Any permitted". Tickets I have found would be issued to normally "Heathrow Airport (Any)".
Same applies to Other airport fare versions which are electronic. Manual NFMs would quote the fare to for example paddington, and then have the HEX/HEC supplement added onto them for use as appropriate but would still give the same fare as the electronic NFM.
Not sure if all any permitted fares would be valid on gat exp regardless, unless they have removed the "Gat Exp only" / "Not Gat Exp" routes. At one time I know all Gat Exp tickets had to include "Gat Exp Only" or "Gat Exp" as part of the route code to be valid otherwise passengers were charged the full fare when on board.