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The GRE measures skills rather than a syllabus: verbal reasoning built on dense reading passages and on words used in context, quantitative reasoning drawn from school arithmetic, algebra, geometry and data interpretation, and an analytical writing task marked on how an argument is constructed rather than on the opinion it takes.
For most candidates out of an Indian school system the quantitative half is familiar ground and the verbal half is not. The problem is rarely grammar. The passages assume a reader who reads a great deal of English prose, and vocabulary is tested in use rather than by definition, which is why a memorised word list tends to run out of usefulness quickly.
It is taken by people applying to universities abroad, usually alongside an English language test, transcripts and recommendation letters, so the score is one part of that file rather than the whole of it. Tutoring here tends to be short and specific: a diagnostic, a few weeks on the weaker section, and full-length practice under real timing, rather than a year-long course.