In my experience of having taken the GRE and actually scoring well enough to glean a 100% acceptance rate re graduate programs, the prep material offered for free online by ETS, (the actual test-makers), was by far the most representative sample of the actual exam.
If you haven’t done so yet, I highly encourage you to download their free computer program, as the GRE has been computer-based for many years now. Take the test following the exact same set of instructions you’ll be subjected to on the day of the exam (e.g., adhere to the strict timing of the exam schedule, particularly noting the lack of break time and sheer cognitive stamina required to take an exam for five consecutive hours, utilize only the onscreen calculator—and/or pencil and sheets of paper—that you’ll be provided with during the Quantitative Reasoning portion, etc). The results are scored and provided immediately upon completion (sans Analytical Writing).
Thus, you can obtain a real baseline for where you presently are vs where you would like to be by exam day, in terms of content mastery.
Then, following adherence to a study plan which accommodates your current practice scores, I’d advise you to take the second full-length practice GRE provided by the program (it has entirely different questions than the first) as you begin to approach exam day. At that point, you can then gauge your level of improvement, and identify any specific remaining areas of weakness you should focus on in the last remaining weeks/days prior to the exam.
In addition to the (HIGHLY RECOMMENDED) free computer program created by ETS (I.e., PowerPrep), the site also provides free study resources, such as a math guide. Here is the link to all the aforementioned materials:
https://www.ets.org/gre/revised_general/prepare/?WT.ac=gre_prepare_170731
For additional free resources, I found the Magoosh website (blogs, tips, daily test questions, additional study materials) and apps (e.g., flashcards) to be very helpful.
