A high MCAT evidences your intelligence and knowledge of the material --
A high GPA shows you can sustain effort and perform well over the long term.
A high MCAT combined with a low GPA says "Really smart, but...?" and your job as an applicant is to answer the "but what?..." question and ameliorate any doubts about the consistency of your future performance.
There's always one or more reasons for low GPA. What was it for you?:
- Immaturity? Simple young and stupid?
- Misplaced priorities? Partying too much or not serious about a career?
- Substance abuse?
- Simple laziness?
- Lack of organizational skills?
- Family problems or interpersonal drama?
- Mental health issues? Depression?
- Culture shock and slow to adapt?
- Adolescent rebellion?
- Learning disabilities?
And there's also the picture your individual courses and semesters paint. Was it lots of B's with a few A's and C's thrown in? A few failures? One or two bad semesters (why?) A slow start? Were your poor grades in the sciences? Humanities? Writing classes? Is English your native language?
Bottom line is the
"Really smart, but...?". question. Answer that to the admissions committee's satisfaction and you're in.