1/ It seems that the taxiway markers have been changed back to the default FSX, rather than the true markings which were in the Aerosoft software...could this be something that GSX did on install? I guess I have to reinstall the Aerosoft s/w?
It's impossible GSX could do this, because it doesn't change anything in the Scenery Library, and doesn't add any new .BGL. The only thing that can cause such thing it's another AFCAD in conflict, but they surely don't come with GSX, which doesn't do any change to any of your .BGLs too.
2/ When I called parking services only appeared "parking 1 to 5", no real gates or terminals
Same as above. If something else (not GSX, because it can't) has replaced the AFCAD supplied with the scenery or created a conflict with it, the parking names will not what they are supposed to be.
Check what AFCAD is currently in use by GSX, by selecting the Customize parking positions from the GSX sub-menu, under "Couatl powered products" menu. The dialog will show the name of AFCAD currently seen as in use by GSX.