What is an example graduate admission grading system? How much is GPA weighted?
Don't bother looking for a magic formula. Everything that is considered by the committee matters, and it's unlikely that you'll ever find someone who will tell you explicitly a specific cutoff for GPA or GRE that will get you in, because you could have a high GPA but lousy letters of recommendation or vice versa, and neither case will likely result in admission.

Moreover, many schools will not operate on such a system, particularly in smaller departments that do not have huge numbers of applicants. I know that I've never used such a system at any of the institutions I've worked at.

So the best advice, trite as it is, is to try to work on everything. It's more important to find someone who's willing to champion your application in the admissions committee than try to "check boxes." You never know what it is that will get you in—perhaps someone's letter of recommendation resonates because he or she was a classmate or personal friend of someone on the committee, or perhaps you expressed an interest in your personal statement that attracted the attention of someone looking for a new graduate student. (And, of course, a high GPA never really hurts.)