The GMAT has a lot more weight than your GPA. I would recommend focusing on the GMAT. However if you're interested in developing a secondary transcript comma I think I would start with MBA Math. The school's really like it, and they send transcripts directly to the schools. It's an online class that you could probably work into your schedule fairly easily.
Given that you arent going to apply for a couple of years, maybe it would be a good idea for you to a some math classes at a reputable institution, post GMAT, however that is more time-intensive and I think once you allay their fears about you flunking out of stats, it might actually be better for you to focus on other aspects of your profile.
How's the community service looking? Have you done any public speaking? What about your hobbies? Keep in mind that they really do look at the whole person and they are not recruiting GMAT scores or GPAs. That said a high GMAT could do a lot of good things your profile.
I would not necessarily rule out top schools because of your GPA. It's really the area you have the least control over, but there are so many things you can do at this point to shape yourself into an excellent business school candidate. I would invite you to review my blog series 1 through 10 on preparing for round one.
Also wanted to mention that the Harvard core program is another opportunity to develop a secondary transcript. One of my former clients did that - had a pretty weak profile and a 3.0 GPA plus he was very overrepresented and he got into Ross and other top schools.
Farrell Dyan Hehn, MBA
Admissions Consultant & Verbal Strategist MBAPrepCoach.com