I think historically Gulfstream were very particular about not allowing their name be used in sims. That is why we tend not to see them commercially at any rate.
EVERYTHING you 'do' on the internet is 'Public'. They WOULD find out (trust me!). What one could do is make a model with the identical specs but name it 'GolfXtreme 3' and maybe you can get away without they complaining (or sueing you). It's the NAME that they are protecting from use (without their permission - which you won't get). It's 'Business'. Nothing personal. Chuck B Napamule