I don't know what you are referring to with "PF3" or "FSS", but the GSX clearance doesn't interact with anything, either the default ATC or any 3rd party utilities you might use.
The GSX clearance it's only a GSX internal thing, which will always work and will result in a subsequent pushback request, at least in a Straight direction, assuming the worse case scenario where you are using a 3rd party scenery which the developer (intentionally, or by mistake), didn't set the pushback preference for that parking.