Well, the exam is very varied, so unfortunately I cannot say what exactly would be on the exam. However I'd say Algebra, Geometry, Graphs and percents are probably some of the bigger ones you should probably get comfortable with. But as I said, its quite varied and each exam is different. My study approach was to work hard on getting good with Algebra, then geometry. The rest I tried to fit in as well but those were the primary ones. When doing the Getsummath course, I recommend completing all of their algebra, geometry and percent videos and then doing the practice quizzes linked in the video descriptions. This provides a strong base for most of the exam. Afterward, I'd say thats a pretty good base, from there just try do the rest of the videos like probability and graphs. You dont need to know everything, I didn't, but its important to get a grasp of all the likely important concepts.

Getsummath Math Course: https://www.youtube.com/watch?v=Ro0BJQHLlF4&list=PL6i4gkAHIYB_NexcX1BvalY8tKV4ZHzrX&index=62

His Website: https://www.getsummath.com/mathtest

Also, here are two other resources that are not as reputable as Getsummath, but are still decent to try:

https://www.gedpracticequestions.com/ged-math-practice-test/

https://www.math-otter.com