This is how admissions works at many schools:
The applications are screened based on gpa or mcat first. That is, there is a minimum mcat or gpa that they require and if you do not make this cut off, then your application is tossed -- no one ever sees your extracurriculars, reads your personal statements, etc. If you are an underrepresented minority, the cutoff may be lower. Whether they look at gpa first then MCAT depends on the academic history of the school. For instance, if they historically have had difficulty passing the boards, then they may weight mcat more.
Usually, there is a formula. For instance, multiplying gpa by 10 and then adding it to mcat score. They then select applications based on the formula to start reading background info, letters of rec, personal statements, etc. Again, depending on the personal idiosyncrasies of the school, they may weight mcat or gpa more or less. If special consideration to minority status, URMs are separated from the rest of the apps and screened differentially.
Obviously, as long as you meet the minimum gpa, then the higher your mcat, the better your chances (and vice versa). But if you do not meet their minimum gpa, then it doesn't help. Some places also have a "must interview" mcat -- that is, since the mcat is the only standardized test applicants take, if an applicant has an impossibly high mcat, then they may get an interview. I do not know which schools do this or what that mcat level is (though, I imagine it is quite high); this is rare, but I have known people who have had letters from adcoms asking them to consider applying to their school (McGill did this a few years back).
There is no way for you to know what the formula at any particular school is unless you have an unethical person from the adcoms committee tell you (they are all required to sign a nondisclosure agreement regarding applicant criteria). The best you can do is to apply to as many places as you can possibly afford with your time and money and make sure you have some "safety" schools -- schools whose gpa/mcat/successful applicant profile matches your own whether or not it is your first choice.