It greatly depends on the course and also on your desired score.
Some courses provide very little knowledge as such and they focus rather on tests, whereas the other ones include the most essential material in the form of lessons. You can always choose the course which will answer your needs. If you feel you need to repeat many things, look for the one that will give you an overview of the basic knowledge and not only teach you the exam techniques.
Generally it is advisable to repeat the most basic rules of geometry and algebra, but really basic (high school). When it comes to the language, you definitely need to be familiar with parts of speech, grammar and punctuation. However, the Verbal part is very specific and some rules that are used in everyday language are often perceived by the GMAT as redundant or unwelcome, e.g. the word "maybe."
