The 2008 came with vacuum assist - more specifically, pneumatic assist. See below, right from the 2008 shop manual. Start taking panels off. Would suggest that you check for proper solenoid and switch operation. Make sure that no vacuum lines have been plugged off. Clutch hydraulic service, etc, etc, etc. And no, that assist kit for the F3 is not backward compatible.
thank you for this info- the diagra mm will be useful till my manual gets here. I did search through the older posts first, but didn't see any threads specific to the GS.