The Math Subject GRE is 50% Calc 1, 2, 3, and Differential Equations. High school algebra and linear algebra are another 15-20% probably. If you do well on just those questions, you will be in the 70th or 80th percentile. Note, this is compared to students wanting to study math at graduate school, so this is very good. So, concentrate on those. But, also learn as much other stuff as you can.
Here is a link to a previous test, including the breakdown of subjects.
Note 25% is "Additional Topics". You'd need to learn several semester courses worth of material to get this stuff. Don't worry about that too much unless you are already pretty good at it. Notice that probability is a subcategory of a subcategory in this category. So, I think you are not quite right on how much probability is on the exam. The point here is concentrate on your strengths. Learning an entire new subject may get you 1 extra question. You're much better off mastering Calc 1-3, Differential Equations, and Linear Algebra. If you are already good at other subjects, then good, practice problems on those too but your time on those should be less.
When I studied for it, I used a test prep guide and I studied a lot of calculus from my calculus text book, for the most part.