The GRE has a brief review of all the math concepts it tests in its Math Review. Stop reading and click on it. It tells us what's on the test - mark which concepts you know and which ones you might not or don't know. Use the Math Review to try to learn ones you don't know.
However, you may need more practice than the ETS Math Review can give you. That's where Khan Academy comes in.
(Note: if you'd prefer a book with everything in one place, I like CliffsNotes Math Review.)
Here's a pic from the Khan Academy website to show you where the video lesson, written lesson, and practice questions are for each concept.
FYI, understanding the video does NOT mean you can do the problem from scratch. So, make sure you take advantage of the practice exercises as well.
Pro tip: If you're not sure whether you are good with a particular concept, try the quizzes or even the unit test (see below pic).
Important note: If you see a concept in the ETS Math Review that you don't know but can't find it on the Khan page I've linked to, type it into the Khan Academy search bar.
BUT if you find a concept in Khan that is not in the ETS Math Review, don't worry about it. I'll list anything you DON'T need in Khan next to each Khan link.
Concepts you DON'T need to know that are in the ETS Math Review: calculate standard deviation, quadratic formula, polynomial division, interquartile range.
And keep in mind your GRE calculator can add, subtract, multiply, divide, and square root.