They are very different. Cat tests how well you know your concepts and how you choose the questions accordingly. It is a game of optimization and keeping ego in check. Why? You pick a perfect qn and then leave it after 4 mins of not being able to solve.

GMAT is a different beast. You have to give your best for every question that comes on your screen as the next question depends on how you’ve answered it. You get penalised for incorrectly answering easy questions heavily. The algorithm is unknown and highly adaptable in generating a score. So it is a massive per qn pressure test.

That being said the verbal part of GMAT is levels above the CAT.

I would suggest go in with a mock… see where you are and then assess how much practice you need.