I have a small tankbag that I am fond of. It is an SW-Motech bag that has since been superceded (I think by the Bags-Connection Daypack II) and I don't even remember the model number. It has been with me for years and has been mounted on several bikes since BMW kept the same tank filler setup for over a decade. The attraction in the first place was that it was not a strap-based bag (not fond of those at gas stops or in general), and there was no metal for a magnetic bag to cling to. It seemed like a great solution. The tankring is replaced by a horseshoe-shaped cleat, and the bag then attaches and detaches easily with a locking pin. The bag itself is a basic clamshell design, but it has reflective piping and logos which still work after years of UV beatings. It is expandable although I confess to never having used this feature, and it has a map pocket on top which has also survived a lot of exposure. The bak does not have the kind of rugged outdoor look you might want for the GS, but that matters little to me as it is functional and that is what the GS is all about. I use this same bag on the R1100S BCR where it is the only form of storage. It has a handle for carrying it off the bike, but be prepared for your buddies to fire up the gay/female joke jukebox if you do ;-)
I was fully prepared to just transfer my tankring cleat to the GSA and continue, but BMW changed the tankring in 09 and now you need a replacement ring from Twisted Throttle before you can attach your old ring. I think I paid more for the ring than I paid for the bag originally. However, I have the option of other bags going forward and it is nice to have old faithful available.