Although I am neither giving CAT nor GMAT, but some time back I did prepared for both and I can put some of the points which I feel might be useful to ppl out their switching from CAT prep to GMAT prep.
First here are some of the threads which would be absolutely useful :
1)
PsychoD's GMAT experience
2)
Some useful GMAT Links
3)
GMAT experience
What is the difference between CAT and GMAT. With a macro-view CAT tests an aspirants ability to adapt to changes and work under stress while GMAT tests an aspirants analytical and verbal skills. CAT uses the mechanism of quant and verbal questions to test the above desired skills. While GMAT uses the same kind of questions to test a persons analytical and verbal skills. Is it confusing?
Okay so to make it more lucid, CAT is more about "the ability to quickly decide which questions to leave and apply logic to solve the remaining questions", while GMAT is more about "the ability to sit through for four hours in front of a computer and solving not so hard questions".
No amount of efforts in CAT prep can ensure that you will crack it on the D-day, a sufficient amount of efforts in GMAT can ensure a decent enough score on the D-day. So is GMAT simpler than CAT? Not at all...just that the preparation strategy is different.
Skills required to crack either CAT or GMAT
1) Ability to not loose patience [keep your cool] on the D-day.
2) Ability to quickly decide the fate of the problem at hand.
3) Ability to move forward forgetting the questions you solved. you can'nt
linger over the questions you answered [in your mind].
4) Practise Practise Practise [Golden words for success]
5)
What differs in GMAT if I preped for CAT :
1) You got more time to answer questions [per question] than in CAT
2) Once you clicked on the answers in GMAT you cant see that question again in your test.
3) You got to write two essays (analysis and argument) and two sections. The essays are graded differently and the score is not added in the GMAT test score. The two sections are quant(37qs) and verbal(41qs).
3) As per the experiences of test takers, the first 15 questions and last 10 questions got more weightage than other questions. So you cannt get these questions wrong. So you need to distribute your time accordingly.
The logic goes like this that the GMAT software assumes that you have an avergae ability before the start of your test. As you keep on answering questions correctly it "ups" your score and as you keep answering wrongly it "downs" your score. Somewhere in the middle of the test it averages out and from then on till the end it just finishes near that. The latest software has some upgrade where if there is lot of variation in the last few questions it just "downs" your score by a large amount...so that why the last few qs are also important.
4) The quant section consists of almost all maths domain [arithmetic, algebra, geometry, coordinate geometry, statistics, trignometry etc etc]. The questions comes randomly and it might be possible that you sometimes wont get any question from one specific field.
5) The verbal section consists of Sentence Correction, Critical REasoning and Reading comprehension. The questions again come in any order.
6) It is easy to sit for two hours in a paper based test. It is very difficult to sit for almost four hours in front of computer even if you are a software engg. [although you get 5 min break between sections..but they are not enough]
7) The sections are fixed in order :
1) Essays
2) Quant
3) Verbal
So the tough part verbal is at the end, when you already have exhausted you energies in the first two section. So practise such that you remain fresh during last hour since otherwise this verbal will become your achille's heel.

Even though the difficulty level is less than the questions in CAT still you need practise to distribute the time.
9) A heavy penalty in terms of score if you leave any questions. So even you got 10 seconds and have to answers 10 questions, blindly tick all the questions. It is considered better to have 10 wrong answers than to leave some questions unanswered.
10) Verbal section is very tricky for someone not familiar with GMATland.
11) The decision to spend "how much" time on some question has to be dynamic and must be taken considering various factors during the exam. In CAT if you cannt get one question in one minute, it is wise to leave than entirely. In GMAT it would depend on lot of things like
1) Is the question belongs to the "royal category" [first 15 or last 10]. You cannt leave it you got to get it right.
2) Are you going ahead of time [You might got easier questions earlier so you got more time at hand] or viceversa
3) Is this from the topic you dont know anything about... no point in spending time if you dont know the basics..move ahead.
4) Is this part of some larger questions like RC... can you get it right by reading the passage a bit again.
12) From the test takers experience, it seems that the earlier you get harder questions the higher your score will be. Ofcourse there is no point in worrying if you still get easier questions...but if you get harder questions earlier on be happy and motivated that you are due for that BIG score.
13) GMAT sentence correction is a lot trickier. It is said that GMAT grammer comes in between classical grammer and the normal common sense. So get accoustomed to GMAT grammer asap.
14) There are two variations of Critical Reasoning (Normal and Bold faced). Learn about them. Also you need a good comprehension ability to crack CRs.
15) Although RC is simple and questions are usually direct (sometimes inferential) but you need to get accoustomed to reading a passage on screen (with scrolling up and down) and answering the questions.
16) Some questions in your test would be experimental and your answers wont be added to the score. Since you will never found out about which ones are experimental no point guessing about them
Oh...its been a long post I guess.. would write more as I get time.
Ketan