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.
MBA Candidate 2015 | Georgetown University
McDonough School of Business