ShashankDave
Hi guys! I know that guys from India prepare for both of these. So am I. Though I have taken the CAT, I havent as yet taken the GMAT, this will be my first time. I wanted to know how different was the experience of the guys with both the tests and are there things about the GMAT in particular that one should be wary of? Also, did the study material for CAT help for the GMAT and vice versa? I would like to know what it will take to get a 720+ plus score for someone who is also prepping for Indian exams. Just share anything you'd want to about these two exams, it will be a great discussion thread and helpful to many.
Cheers!

My 2 cents. I have a 750 on GMAT (2015) and had a percentile of 99.3 in CAT (2008, the last paper based version before it went for computer based).
CAT for me was given with not that great preparation (stop and start kind of preparation) but GMAT was given with proper practice and preparation of 3 months. A caveat, my differences will be based on the paper based CAT and not the computer based one.
Logistical/overview of differences:1. Biggest difference is that you can skip the questions in CAT while the GMAT is a computer adaptive test and as such you must answer every single question. The next question is based on how you answered the previous question.
2. Scoring is different (+3, -1 for CAT), while there is no such set marking for GMAT exam.
3. GMAT has 4 sections spread over 4 hours (with 2 8-minute breaks) while CAT had a 100 question exam in 3 hours (no break).
Technical differences/similarities.1. Quant is on the simpler side (for concepts tested) for GMAT than quant in CAT. GMAT tests concepts from 9-10th grade schooling in India but the application part is a bit tricky. CAT quant preparation will be an overkill to a large extent.
2. DI/LR section in CAT is similar to the IR/CR/DS (to some extent) sections in GMAT respectively.
3. Verbal in CAT was a bit direct application of rules in grammar and vocab while SC in GMAT is a whole different ball game. Meaning as well plays a big role in SC. RC for CAT was comparable (I had 99 percentile in verbal in CAT) and so was my RC score in GMAT (47 raw score, 95 percentile as per the GMAT ESR). Again, RC level might be a bit harder as the material deals with humanities and other esoteric topics (related to USA, such as women suffrage, topics related to works of Martin Luther King, history of USA etc) that Indians might not know about. This is where you will have to train your mind to assimilate foreign information under the time constraint. CR is very similar to DI/LR section of the CAT. If you did well in those 2 sections in CAT, you will have a better understanding of CR in GMAT.
Summarizing, the understanding portion is very similar (NOT SAME) and you can transfer some learning traits from CAT to GMAT. But these 2 have certain differences (mentioned above) that make it worthwhile to spend 2-3 months on GMAT preparation if you are also studying for CAT.
Feel free to ask questions/PM me if you want for clarifications on points mentioned herein.
I think
kinjiGC gave his CAT after 2009 and thus will be able to provide more inputs on the differences.
Good luck.