If you are fitting the original Vario case on a regular GS you'll need only a Torx wrench or two (IIRC T30 and another smaller). You will need to take out six screws that fix the original piece to the bike, loosen a couple that hold the pillion handle under the seat, lift the whole thing, exchange the original support for the adaptor, and put the screws back on. Pretty straightforward, no other issues.
I don't have a clue about the GS Adventure. It uses a different kind of support.
Sent from my iPad using Tapatalk HD