I think you can fit the ADV luggage to the 1200GS but you will need the hardware to go with it, ADV pannier frames,ADV luggage rack to replace your original and there's a additional mount for the top box which attaches to the luggage rack,and then there's the panniers and top box
from a BMW dealer it won't be cheap but you may get lucky and find a used set on here or e-bay, good luck