What would I be able to expect on the actual Mathematics GRE?

I have purchansed a book "GRE Mathematics Test", it has 6 full length exams. It covers what is on the ets website, you know stuff like algebra, calculus, linear algebra, analysis, topology, combinatorics, logic, etc... but I get freaked out by the "this is not comprehensive" note on ETS and in the book... I am confused on what else to study.

I haven't taken the practice tests yet in the book because it just arrived in the mail but hypothetically, if I were to be able to answer every question on all the practice tests (you know if I have a comprehensive knowledge of all the subjects covered in the book), what would I be able to expect on the actual GRE test? How much of the GRE questions cover topics not part of subjects reviewed in the practice books?

I would love to hear from people who have taken the actual GRE subject test and also reviewed from the books as well

thanks!