this is simply my opinion which is based off of my own understanding of the situation and how things go so take it with a grain of salt.
First, no matter if you are applying to allopathic or osteopathic programs, you should generally not be taking humanities classes to "boost" gpa but be focused primarily on upper division science courses that would be more representative of the coursework in medical school. Medical schools like observing trends and most of the predictions that they do are based off of your 4 years coursework and your MCAT to gauge how well you will perform in medical school. Whether you will become a good doctor is based off your interview, personal statement, LORs and your E.C. experience. Both are needed. What they would probably not want to see is simply taking a year off to take classes that are not relevant to your field of study (medicine) in the future simply to cushion gpa.
Are you planning on applying allopathic or osteopathic?
If osteopathic, AACOMAS is more forgiving and you will most likely benefit more from cost/time if you take a year off to retake classes that you recieved Cs in for better grades with their repeat grade option when entering coursework and calculating GPA. If allopathic, try to take upper division science courses that you have not taken before, ie. if you did all the medical school prereqs look to take classes in neuroscience, anatomy and physiology, biochemistry, genetics, etc. to prove you can handle an advanced science course load.
If you are uncomfortable with informal post baccs, I'd suggest a formal post bacc at a COM that offers a 1 year program in upper division sciences which are even taken by 1st year medical students. An SMP is usually considered only when you believe you have a serious deficiency in one regard of your application (gpa usually) and that to boost it any other way would not be time efficient. SMPs work both ways from what i have come to understand, it definitely has the capacity to boost you into medical school if you fall into the non-competitve spectrum, but at the same time, if you perform simply average or subpar, it's an axe in your application. It's also expensive and often takes 2 years. If you find a right one with a linkage into the medical school then I believe that would be worth considering if you have the financial and motivational means and discipline to do so.
Good luck.
