If you are required to have GRE is not sure on how to prepare for it, you can take some courses from Coursera to prepare you. The main three focuses that GRE evaluates on you on are Reasoning, Math, and Writing. Coursera provides a specialization called Reasoning, Data Analysis, and Writing. Although you have to pay for the specialization certificate, you can also audit each of the courses individually for free. Providing you with an extra source of training for the GRE.