BMW K 1600 GT
31.89/32.68" (810/830mm) standard
30.71/31.50" (780/800mm) option rider seat incl. pillion passenger seat
30.71" (780mm) special accessory seat, single-section
29.53" (750mm) special accessory seat, single-section
BMW K 1600 GTL
29.53" (750mm) standard
30.71" (780mm) option seat, single-section
30.71/31.50" (780/800mm) special accessory rider seat incl. pillion passenger seat
31.89/32.68" (810/830mm) special accessory rider seat incl. pillion passenger seat
So the difference between the Standard GT seat and the Standard GTL seat is 2.36" (60mm) with the GT seat in the Low position, or 3.15" (80mm) in the High position.
I don't know exactly how far down & forward the footrests are on the GTL versus the GT. But BMW does list pretty much every change (including seats, footrests, trunk, radio prep, etc.) as options. So theoretically you could put the highest seat on a GTL, or put the lower footrests on a GT.
Good luck getting that through special order on the first batch of bikes, when demand is still high and availability still limited.