I group the GMAT and GRE together because they’re very similar tests; I’ve been teaching people to do well on each of them since 2002. Most of my GMAT and GRE students come to me with particular subject areas in mind so we’ll probably want to talk about the details of your situation in order to make a plan. Of course I can help on all sections of these tests, including the quantitative (math) and verbal portions, and even on the writing sample (which is a pretty straightforward section on which to improve) if you need it.
For years, if you wanted to go to business school you took the GMAT; it was the only game in town. But now many business schools accept either the GMAT or the GRE. Given that, which one should you take? I answer that question1 and many others in this video interview.
Note that a similar problem faces those going to law school in that many law schools now also accept the GRE, but so far the LSAT remains a practical necessity for most, as a majority of schools still require it. I therefore strongly suggest that law school applicants take the LSAT (which also happens to be a much better test of fitness for law school.)