It is also appropriate to call attention to non-academic circumstances which may have prevented your GPA to accurately represent your capacity, so as to place your record in an appropriate context. If circumstances affected your performance, you can describe them and indicate what steps you have taken to overcome those circumstances. The important thing is to phrase your explanation so that it does not sound like excuse-making. Having others read and critique your statement can be very helpful here. If your GPA was the result of a learning disability or other such condition, you will have to decide whether or not to disclose the disability in explaining your academic performance.
Federal law prohibits discrimination in admissions on the basis of disability.
It is even illegal to require applicants to disclose disabilities on an application form. But many students feel that disclosing their disability in the admissions process places them at a disadvantage. I can't argue that they are wrong. If you show that reasonable accommodations put you on an equal footing with the other applicants to the program, the program can't disqualify you on the basis of your disability. If you don't have an undergraduate GPA because your school doesn't give letter grades, the other criteria assume greater importance in the admissions process.