There are a few ways to go about post-bac coursework.
The first, as you have alluded to, is a traditional master's program. From what has been shared by AdCom members on SDN, a graduate GPA is not going to have a huge impact on your competitiveness due to historical grade inflation. However, this option will make you more marketable should you not end up getting into med school.
A second option would be to pursue more undergraduate coursework, either on your own or in a structured post-bac (UPenn, UConn, Temple, etc). The benefit of this route is that your grades get lumped in as undergraduate coursework and are more heavily weighted than graduate alternatives. The downside is that students tend to dry up federal student loans in their 5th and 6th years, so much of it is funded out of pocket or via private loans.
The last option, and this one's a hail mary, is a special master's program or SMP (Georgetown, VCU, Cincy, Tulane, etc). These are basically 'med school tryouts' where you take the same exact courses as the medical students (except fewer of them per semester) in order to demonstrate your ability to handle the curriculum. The benefit is that many of these programs offer guaranteed interviews or conditional acceptances if you maintain a certain GPA in the program (and end up with a decent MCAT). The downside is that these programs tend to be very expensive. Also, if you don't do well in them you can kiss your chances of ever getting into a med school goodbye.
...hope that helps.
For a much more detailed description of this summary check out:
GPA Enhancement Programs
and
The Official Guide to Special Masters Programs