I was curious about this too, and did some checking at
www.kyb.com . It seems that both the GR-2 and the Gas-a-just are both gas charged, but with one difference I found. The GR-2 has the gas and oil mixed in the shock which I guess can foam under "spirited" driving. The Gas-a-just has a piston to keep the oil and gas separate to prevent foaming.
I picked up a set of the Gas-a-just shocks myself, though i won't be able to test them until monday.