I've never seen lowered foot pegs for the rider on a GS, I think Wunderlich do pillion ones.
You might be able to swap the Adventure pegs with a different model, perhaps the R1200R are a bit smaller and therefore lower down, but you will lose the nice big platform.
Mike
Fastway rests will lower the pegs by about 10mm, the TT ones don't fit they said the would but when I fitted them the angles were all over the place, ( I have pictures if anyone doubts this) they refunded me.
My seating position and legs felt better when on the suggestion of the dealer here put the front of the seat on the high position and the back on the lower one