You will need along with the panniers themselves the pannier rails which fit no problem to the GS.
If you want the top box too you will need the mounting rack from a GSA. It all fits a GS, i know as i have done it.
I was standing on my head trying to do the tummy rub and head pat exercise.
The right-hand side case frame clears the tyre by 115mm. On the left, you could comfortably use an oval exhaust can 100mm wide or even a stacked pair. It would need a special link pipe, support bracket and an outlet tube to feed hot gas away from the number plate.
I have the Akraprovic "shotgun" system which uses 100mm cans. I would quite like to reposition the cans but they system is titanium so I've left it well alone. Before finding that system I did talk to Andy at Extreme Stainless (Hixon, Stafford) who was up for making a Y joint to carry two 100mm cans.