I definately agree with taking the classes at a university. Otherwise it will look like you are trying to take the easy way out.
There are different options to raise GPA after college, you could do an informal post-bac program (do-it-yourself), a formal post-bac or an SMP (*special masters program).
The first 2 are undergrad courses while the last is a graduate program. The draw back of the SMP is that it does not change your Ugrad GPA but does show that you are able to perform at the med school level. So if you have a borderline GPA (or have a completely terrible GPA) SMPs are excellent options.
So an SMP can push you over the edge if you are close or can show that college was just a fun time and you are ready to work now.
In terms of timing, if you start a post-bac/SMP you would most likely be placed on 'hold' by the medical school until the first semester grades come in. So unless they flat-out reject you, they will see your grades.
A lot of programs have linkage while others have 'understandings' with certain med schools.