I too find same. I have gone through individual sets of MGMAT. Questions are really easy. So now I am solving problems from Quantitative Aptitude for the CAT by Arum Sharma. Though this book is meant for Indian MBA exams, you can refer this. Questions are really tough. But I am not sure whether this book is available outside India. You may try.
I believe that GMAT Quant and CAT quant is significantly different especially when it comes to topics covered. While this strategy could pay off, I would recommend you to make sure that you double check that the topics you are doing from books for CAT do appear in the GMAT. Check against the Manhattan guides for the range of topics covered.
All the best!
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