If you have the OG, I would steer clear of Grockit as in many questions all they do is change slight details of a question from the OG and repost it. This will not help you improve and may give you a false sense of confidence as all you will be doing is repeating questions. Besides this expect a lot of repeats, when going through questions. Although the interactivity of the platform is nice, it tries to mask too many flaws. It has potential but not enough original gmat like questions. I would save your money.
Hope this helps
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