I don't think the Grockit CATs are very reliable.
I have taken 2 till date and both have been considerably harder than any other CATs, including Kaplan
One trend that I did notice was that Grockit CATs tended to post questions which they knew you had already gotten wrong or bookmarked. Which is not really very intuitive.
I would suggest Grockit for other reasons like tracking your overall progress and strengths and weaknesses but not really for their CATs.