As far as I am aware the GPF was only fitted from around WLTP time late 2018 onwards.
I also believe the GPF fitted cars have a louder exhaust burble and crackle. V nice!
I may be wrong on the above.
However, if you want to see how effective GPF is, go compare exhaust tips on GPF cars vs previous non GPF cars.
The pipes on my car are almost as clean as the day I picked it up new and I never clean them.