I just put the rack and panniers from a GSA on a friends GS over the weekend and ran across a few little surprises so I thought I'd post this for anyone else that was thinking of doing this.
It all mounts pretty easily, I'd say it took less then a half hour, once we had everything we needed together. And it should go without saying but I'll say it anyway, first make sure they are off the same style bike, ie. the muffler on the bike they came off of is on the same side as the bike you want to put them on.

If you have a GSA you're taking the racks and panniers off of, SAVE EVERYTHING, including the passenger foot peg mounting brackets. The hardware mounting kit, 8 bolts and 4 spacers, from BMW is $179 and you can't buy the pieces individually so don't lose anything and the kit doesn't include the foot peg mounts.

If you'd like to mount the boxes on your GS make sure you have everything. It can add up fast if you have to go out and buy any parts. Again, they mount pretty easily, the hardest part was, we needed to remove the muffler to change the passenger foot peg bracket on that side. The lids of the boxes rubbed on the passenger grab bar when opened so we changed that with the one off the GSA too. The tops would open, you just have to pull them away from the bar. The other thing I'd look out for is if you have a top box already it may interfere with the pannier lids when you open them.