are you riding to Greece or shipping the bike? The tarmac in Greece is a lot more slippery to here. I would think a little more slippery than UK wet. The dirtroads vary from Acropolis rally ruts and boulders to pretty perfect wide gravel. There is no sand unless you ride on a beach..
GSes are better bikes than any road bike for greece but are not perfect. Too heavy, esp the Adv. Thats why most people down there who actually ride in the country and on dirt roads regulalry are on shagged XT600 and old TDMs.. ah and the fact that a GSA is over 20k eur...!
Latest trend in Athens by my observations seems to be the (highly unsuccessful here) Suzuki Vstrom 600/1000. Seems like at least 50% of the non scooter populatioon are those Vstroms... Maybe Suzuki give them for free..