I break the process into two parts as follows:
Part I:
There's no one guideline or standard as to who to select for the interviews. Some programs weigh heavily on GPA, while others don't really care if you have above a certain GPA (e.g. >3.0). The programs usually factor in other extracurricular activities, rec letters, essay, leadership and job experience, etc. They rank you just like how they rank you for the match, and say the top 20 will be invited for interviews. (There are exceptions: some programs will interview you (first come first serve) without even looking at your app package.)
Part II:
Now the interview part. Some programs will take in consideration of your points from Part I, but some don't. For programs that don't, you're starting over again and how well you do during the interview will get you ranked high.
My advice: if you have low GPA, apply to many diverse programs as you can (e.g., 7-10 programs, teaching to community hospitals, rural->non-rural, etc. and of course only the ones you like) since you don't know what their criteria are for interview selection. You'll end up with some invitations, and usually these programs weigh less on GPA or if you have special connections with the programs. Now the next part is just to be well-prepared for the interviews.
Really, don't just try to get the best GPA, but make sure you're well-rounded (e.g., volunteer, leadership, making connections, etc.)