Hi Alex,
I have taken the GRE and honestly it's nothing to worry about. The quantitative section test your ability to spot a problem rather than your maths skills. The math involved is the stuff you would have learned in the early stages of high school, but if you are still worried then ETS ( the board that prepare the GRE) have prepared a book of example papers and pretty much every type of question is in there. (You can get it cheap on Amazon).
Plus, they let you use an on screen calculator during the test!
Hope that helps you!