Definition of "sub-par" -- performance below a standard or average level of performance.
Par (for med school admission) -- 3.66/32. Anything below a 3.66/32 is, by definition, sub-par.
If you mean at what point is your app unacceptable...well, that depends on far too many factors to count. That being said, I'd say below a 3.4 GPA puts you pretty much out of the running without some extraordinary qualities, MCAT score, LORs, and such elsewhere. I say 3.4 largely b/c that is the avg for an applicant and <50% of applicants are admitted. In other words, if you're below a 3.4 GPA, you are definitely in the "generally not accepted" stack (GPA-wise, anyway). The same could be said of a <28 MCAT score. (Of course, people do occasionally climb their ways out of the "TO BE REJECTED" bin, but it's relatively uncommon and requires that someone w/ better stats showed him/herself to be a weaker applicant overall).