Different Rear frame. Different part numbers, same low price.
I read several places that the GSA has a stronger rear subframe to deal with the increased loads and rough roads of overland travel. I assumed that this accounted for some of the weight difference I couldn't add up.
And it has to straddle that 24L tank.
FWIW (little), my '15 800GSA frame is find after getting a fair hammering from Dallas to South Chile, but all is well. Can't say same for bent front rim (large rental fleet here all sport Touratech Excel front rims from day 1).
My ZFE just went ZF-OUT in Puerto Montt, but that's another story...