How different is the preparation for CAT and GMAT, in terms of subjects and topics?

Yes, preparation and exam pattern for CAT and GMAT are quite different. While CAT is computer based test, GMAT is computer adaptive test. The syllabus and question types also vary a lot. While conceptual clarity is common for all exams, specific exam taking skills would vary. In case you are planning to appear for both exams, you have to prepare separately.