Interrogating fundamental cognitive abilities:
The GMAT may appear to be a Maths and English test, but it's not really. The test poses questions within these domains in order to probe more fundamental abilities, notably Pattern Recognition and Analytical Reasoning.
It applies pressure:
None of the questions are impossibly difficult – given sufficient time all can ultimately be resolved. But the time pressure exerted by GMAT is a design feature, deliberately intended to exert stress on candidates. An easy way to readily pick up some extra points is to exercise rigorous timing discipline. You cannot get a good score if you miss or rush the last few questions. So, divide the test into quadrants so that you can check progress/modify your speed at the 20 minute/10 question, 40 minute/20 question and 60 minute/30 question waypoints.
It finds your weakness:
GMAT has been around for decades, and the question-masters know intimately every cognitive weakness and reasoning defect to which candidates are prone. Diligent practice reveals that GMAT's range of ploys, while devious, are necessarily limited.
It punishes carelessness:
Carelessness is a killer, no matter how smart the candidate. So, the last step for every problem: look back and read the question one more time, to ensure that you answered the question as stated, and did not drift off-target.
No more than at 10th grade Maths and a similar proficiency in English is required. But the questions that GMAT asks are much more sophisticated than 10th grade.
What aspect of the syllabus is being tested ? What are the typical pitfalls ? Is there a more agile, non-obvious approach? What must be true but has not been stated ?
Don't waste time on elaborate computations – if your scratch paper is covered with numbers, you're doing it wrong – your ability to emulate a $5 calculator is not what GMAT is testing.
Is crucial. There is no credit for "almost right". Practice will allow you to gauge your maximum speed, which is when you are surfing just on the safe side of an "accuracy catastrophe".