I asked my contact @ McDermott about it. This is the reply I received:
"The G-core shaft falls in between our standard maple shaft and I-series shaft. The G-core incorporates the same carbon core technology as the I-series shafts, but it is only in the stroke area of the G-core shaft (it does not run the entire length of the shaft, as the I-series does.) The I-series shafts are still the premium shaft we have."
So if you're already using a McDermott I-series (1, 2, or 3; I have the I-2 and really like it), I don't see any real reason to bother trying it. Maybe if you're looking for a new shaft, but I'd say spend the few extra bucks and get the I-series or one of the competitors that are out there. But that's just me.