Mark 1 and Mark 2 GS's appear to be essentially the same with many common accessories being transferrable e.g. exhausts, seats, screens, cylinder head protectors. Think tank shape changed; tankbags may not transfer. Relatively easy if buying Touratech or Wunderlich parts to check their on-line catalogues for model compatibility.
The 2010 GS (Mark 3) has a couple of differences which mean some accessories will not transfer from my 08 bike e.g. BOS exhaust and Touratech cylinder head guards. Probably be offering these items (plus a Nippy Normans high wide tinted screen, and a BMW tankbag) for sale in the near future either on this site or e-bay.