There is preload (hand screw thing at top) and rebound (little screw at bottom of shock. visible from right hand side) on a GS 12 rear shock.
Best settings, with tyres at 27 front and 33 back (Michelin - Anakee's) is
preload ( 5 1/4 - 5 1/2 turns in from hardest - i'm 90 kgs in weight. )
and
the rebound - bottom screw 1 and 2/5 ish out from hardest. Have the 'slit' of the screw in between the "R" and the "E" of the word rebound.
This will give good feel, as well as making the back just hard enough from falling backwards, which in turn, will make the front feel a whole lot better.
This setting is also good, as it makes the bike feel balanced when going over bumps. i.e. you dont get the sensation of the front wheel going over a different size bump to that of the rear.
Hope this helps...