I will admit to being somewhat puzzled when I have seen these CAT versus GMAT questions come up, since I have interpreted CAT as computer-adaptive test, of which the GMAT is an example. After doing a little research into the matter, I now understand that this CAT refers to Common Admission Test, which is also computer-based but is held in India for admission to business administration programs in that country.
An overview of each test provides some insight:
Verbal Ability and Reading Comprehension (VARC): Critical Reasoning, Grammar, Analogies, Reading Comprehension
Data Interpretation and Logical Reasoning (DI & LR): Tables, Graphs, Charts, Data Correction
Quantitative Ability/Aptitude (QA): Geometry, Trigonometry, Word Problems
A complete syllabus for 2019 can be found at MBA Rendezvous.
Verbal: Reading Comprehension, Critical Reasoning, Sentence Correction
Quantitative: Problem Solving, Data Sufficiency
Integrated Reasoning: Tables, Graphs, Charts (multi-source reasoning)
Analytical Writing Assessment: Analysis of an argument
There are plenty of other differences between the tests--their cost, the schools that accept the scores--but in broad terms of content, that should prove somewhat helpful.
Whichever test you choose to take, good luck. I can only speak to the GMAT, but I imagine the CAT is challenging in its own right.