Thank you for your question. With a low GPA, taking courses can be a very good way to prove that your more current academic experience is strong and to show that you can commit to and achieve a goal, so I applaud your commitment to doing so. I think which certification you pursue depends largely on your overall profile and the areas where your grades are weak. For example, if you did not take finance classes in college, or if you did take finance classes but did not do well in them, then a certification focused on finance skills, rather than, for example, one on project management would do more to offset the admissions committees' concerns.

You should also be sure to write the optional essay (usually phrased as "is there anything else you would like the admissions committee to know") to explain your low gpa and guide them towards evidence of your abilities, like a high GMAT, courses or certifications, etc. Don't make excuses, but offer a reason for the low grades. If there are positive stories within the low gpa - like higher grades in business classes or in your last two years for example - tell them that as well.

Good luck!