Which math classes should I take?

The problem you will face is that the GMAT does not have any math that would be strictly considered "College" level math. In fact, most of the math is highschool or lower, and that is one of the toughest parts. This test is the advanced applications of low level math. The test won't have calculus on it, and the hardest concept is probably statistics. Instead of taking math classes at your college for the GMAT, I would recommend spending time outside of school studying strictly "GMAT math". It is a whole different beast, and will not come inside of any college level class. Look into the Quant forum on GMATclub and see the books that are out there, but the best in my opinion come from Manhattan GMAT. There are 8 guides in total, and 5 of them deal with EVERY POSSIBLE math concept you could come accross on the test.

Instead, I would focus your time at school taking a couple math classes that will beaf up your application when you do decide to apply. The two best classes will be calculus, and statistics. If they offer either of these classes with a "business" spin then that's even better (aka. "statistics or business decision making" or "business calculus" or "managerial xyz" etc...). You just have to make sure that you get an A in this class, and if you think you will get anything lower than a B or B+ I don't even know if I would take them at all. I would work to get a Q48 or higher on GMAT quant to prove your quant ability.

Everything you need to know is out there, but your college isn't going to teach you GMAT math. Good luck to you, and look around GMAT club. If you could soak in every piece of information on this site, you could easily score into the 99%. Easier said than done

PS. If you take statistics, you might even use a few concepts on test day! I had 2 stats questions on my test... you really only get them if you're doing well on test day, so lets hope you get a lot!