It depends on your undergraduate background. If you had no calculus or statistics in your undergrad curriculum, then taking one of those classes could be good. Otherwise, consider taking the GMAT again.
Or, consider doing both! You obviously want to maximise your odds of success, and it sounds like you're giving yourself a lot of time before you apply, so consider taking the class and re-taking the GMAT one more time (after some more studying).
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