My disclaimer: I'm a PGY-2 so my GMU experience is from the late 90s. I did a "do-it-yourself" post-bacc at GMU and found the classes were **excellent.** How many are now available as night classes, I dunno - when I did it, only gen-chem was offered at night. (I was able to take physics and o-chem first thing in the morning, though, which could also be balanced with a job.)
GMU is a strong school that is getting better... but it is still a commuter school with lots of part-time students and people who come in and out while they balance life, money, work and school. I don't think you can compare traditional undergrad stats on things like retention and graduation rates because the populations of GMU and American are very different.
Given that you're going to go into debt (presumably) to go through medical school, trying to save money on your post-bacc is a fine goal and one that's understood by AdComs. If you are a self-motivated person who can push your own goals while doing your post-bacc work, I suspect you can do just fine at GMU.