Hi skrr.
Here's the deal.
The purpose of preparing for the GMAT is simple, and it's not to learn a whole bunch of stuff and prove that read about it. The purpose is to hit your score goal. So everything that you do should be driven by that purpose.
Generally hitting a GMAT score goal takes being familiar with the building blocks that the GMAT uses when creating questions, the concepts, and being good at figuring out answers to the questions.
For all you know you already know most of the concepts, and your skills in getting to answers may or may not already be sufficient for scoring close to or hitting your score goal.
Given all that, your best bet is to do some preparation, so that you have some idea regarding what the test is like and what's on it, and then take a practice test, preferably one of the official GMAT practice tests that are contained in the GMAT Prep software available for download here.
https://www.mba.com/us/store/download-fr ... tware.aspx
There are actually some review questions in the GMAT Prep software, along with a basic review of most of the math concepts that appear in the questions.
Once you have familiarized yourself with the basics of the content and have done 10 - 30 questions of each type, maybe taking a week or two to accomplish that, then you could take a practice test to see where you stand.
THEN, you can look at the results of that practice test to determine what you have to do in order to hit your score goal. You may need to do a lot or you may not need to do much. Whatever the case may be, by getting warmed up for a week or two and then taking a test, you will have set yourself up to determine what you need to do.
For some more details on how to score above 700 on the GMAT, you could read this post.
How to Score a 700+ on the GMAT - A Mini Guide for Success