GSX reads the Parking Type ( Small/Medium/Heavy ) from the scenery AFCAD, but if a GSX customization exists, it will always take precedence over the underlying AFCAD so, even if you change the AFCAD, GSX will read the .INI file, assuming it was made for *that* AFCAD.
Not that this has any effect on GSX or the simulator itself. The simulator will always use the AFCAD data, since it doesn't know anything about the GSX .INI file. In GSX, the parking Type only affects the kind of operators chosen ( Cargo or non-Cargo ), but the Small/Medium/Heavy doesn't matter, the only thing GSX reads is the actual parking radius.