Broadly speaking there are 4 skills that are tested during GMAT.

The GMAT primarily tests :

Endurance and ability to focus
You’ll have to stare at a screen intensely and focus for nearly four hours. Keep this in mind while taking practice tests. Get used to work for many hours & in the end learn how to relax. The physical and mental exhaustion is part of the test’s challenge.

Basic knowledge of grammar, math, reasoning, and argument formation
The second skill, a basic knowledge of grammar, math, reasoning, and argument formation. No calculators are allowed on test day, so you need to practice doing basic math calculations.

Test-taking skills
Ability to guess, working at an appropriate pace, and making decisions under pressure. Timing is a major part of test-taking skill, particularly for the GMAT & CAT.

Problem-solving abilities
To improve your problem-solving abilities, the fourth skill, we have extensive information on reasoning techniques and math concepts.