I'm not very sure how Grockit CAT fares against GMATPrep or the actual GMAT.
I took the GMAT Prep a couple of months ago and had scored 660 and I took the Grockit CAT last evening and scored a 710 (Q45, V40)
although I couldn't complete my Quant section and had almost left around 5 to 6 questions unanswered. The reason for this was that DS questions were lengthy and most PS questions required intensive calculation. The level of verbal questions was good as well.
Giving +1 kudos is a better way of saying Thank You