It comes down largely to background skills. What do you need to know before you start studying for the GMAT test? What don’t you need to know?

Quantitative

For quantitative, you’ll have to dust off your basics from school, up to first-year high-school algebra and geometry. That’s it; you’ll never need — or even get the chance to use — anything more ‘advanced’, such as trigonometry, calculus, or formal proof.

Particularly important are arithmetic (no calculators!), basic algebra, fractions, percents, ratios, number types (odd/even, positive/negative/zero, prime), inequalities, quadratic equations, and basic formulas for angles, circles, rectangles, and triangles. 

On the GMAT test you’ll need to execute these skills consistently, confidently, and efficiently. Don’t worry about training speed (which will naturally improve as you gain proficiency). Instead, focus on intuitive understanding.

For every step you perform, what do you need, what does the step do, and what results will it give? Can you write a basic problem that requires it? Especially for percents and ratios, can you make up real-world examples?

If you practice from textbooks, stay focused on the main batches of problems, avoiding ‘challenge’ or ‘enrichment’ items. Note carefully any exceptions to rules or patterns, especially if they affect whether certain information will solve a problem. For instance, two equations usually determine the values of two variables, but not if those equations are multiples of each other.

Verbal

For verbal, the only necessary skill is adequate proficiency in reading English text.

As you read a passage, you should be able to explain what it says, in your own (informal, conversational) words, in your native language.

If you can consistently do this, you’re ready for the GMAT verbal section. If you can’t, shelve the GMAT for now and use your study time to read articles from well-written sources (e.g., New York Times, Harvard Business Review). As you go, check your ability to summarize the text as just described. Don’t time yourself, and don’t rush! You’ll gain speed as you gain skill — but if you force a faster pace than you can manage, you’ll gain neither.

The same regular reading will give you a sufficient vocabulary for the GMAT, which depends only on words that a typical educated adult knows, and neither requires nor rewards any knowledge of ‘advanced’ words or specialized jargon.

If you encounter an unfamiliar word in an article, figure out as much as you can about it through context clues and careful, logical thinking. Do look up the word(s) to confirm your understanding… after finishing the article! The same on-the-spot consideration should handle exotic-looking words on the GMAT; you needn’t study vocabulary lists.

Don’t forget that the goal of reading external material is to develop your reading comprehension and vocabulary to levels that are adequate for for graduate business school, not to prepare for the GMAT itself. You cannot improve at GMAT Reading Comprehension by ‘practicing’ on external articles!

Such articles do not come with GMAT-style questions, nor do they conform to GMAC’s extremely strict stylistic and formatting regulations. If you restrict GMAT-style reading and note-taking to actual GMAT passages, you’ll develop a better filter for the information you actually need.

Even for Sentence Correction, the same baseline reading skill is the only prerequisite. GMAT mostly tests issues that you won’t need to study, since you’ll already understand them (e.g., subject-verb agreement), or else that you can’t study, because they’re functions of context and common sense (e.g., differences in meaning).

For the few things you’ll genuinely need to learn, stick to GMAT-specific sources. Don’t use general grammar books, and don’t study grammar terminology beyond the names of basic building blocks (noun, verb, subject, clause, etc).