It's good to see you're ready to study for the mGRE. Unfortunately, the mGRE asks such a wide variety of questions about different mathematical topics, that it is not very easy to "study for the test" as you say people have been telling you to do. I can only say that it would be very beneficial to have a deep understanding of calculus; that is, knowing basically every theorem from calculus I,II, III, and vector calculus and how to apply those theorems. The thing with the mGRE is that there is almost always a trick or some theorem you can use to quickly solve the problem. Therefore, memorize useful nonobvious theorems. In each subject, there are a few theorems that stand out as being most important. For example in Algebra, Lagrange's Theorem and the Fundamental Theorem of Finitely Generated Abelian Groups can be used to solve a lot of problems on the mGRE, at least in my experience. Another difficulty of the exam that you can never fully prepare for is not knowing what types of questions will show up the most on the exam. I took the mGRE before I had taken Abstract Algebra, and it seemed that a very significant percentage of the exam concerned abstract. I've heard that some people take the exam, and it has tons of linear algebra or Complex Analysis. Nonetheless, you can know for sure that 50% of the exam will be about calculus, so be very solid in that area. My recommendation is that in addition to the study of Calculus, be proficient in your knowledge of Linear Algebra, Abstract Algebra, and Analysis. Those seem to be the areas which would give you the most bang for your buck in terms of study time for results. There will inevitably be some funky questions in complex analysis, number theory, geometry, or probability, but knowing what to study to anticipate those problems is near impossible.
I will say, though, that on many of the practice exams, the type of probability question that comes up the most concerns variables which have uniform distribution. I will not divulge any of the types of questions I encountered on the actual exam, but I can say that truly anything from any subject at the undergraduate level is fair game......anything.