I've done the Mt. Greylock century twice. I just used compact gearing (34x25). As said above, the route climbs Greylock via the easy side and descends the steep side. I did the ride before they repaved the road; that descent off of Greylock used to be pretty scary as the road was in horrible condition.
The worst climb of the Greylock century is the third climb of the ride. First is Greylock, the second is climbing out of North Adams on Rt. 2 heading east, the third is some back road that you turn onto off of Rt. 2 that feels nothing short of vertical. And then you're half-way!
