KYB does say they are identical. A very reputable company (Turn In Concepts) has proven this to be false. They have opened them up to see for themselves. Many people can not tell the difference between them in how they drive. Some people complain that the Excell G are too harsh with poor ride characteristics. As far as build quality they seem very similar.
I am currently running Excell G front and GR-2 rear in my wife's Legy Outback. Overall we have no complaints. I will admit the fronts are more harsh but not to the point where I would complain.
The GR-2 is well regarded but KYB in all their wisdom has decided to discontinue the better of the two.
Many people are turning to Tokico HP (blues) instead of the KYB. It does not have a wagon fitment but that can be taken care of easily by slotting the struts or adjustable strut tops.