I also have on '07 GS.
I started off with the older style BMW tank bag which was designed to fit the bike. But found it ugly and too big. It attaches using a hook at the front and a zip to an adhesive pad at the back.
I replaced it with one designed for the '08 onwards bikes which is smaller but meets my needs perfectly. Although its not waterproof. I was glad to find it went straight on so I didn't have to remove the adhesive pad from the previous.
Going the other way doesn't work - the older bag won't fit the newer tank shape.
Then again maybe look at the Wunderlich one?