General rule of thumb: you have to outperform the GMAT if your GPA is below the average (and even more so if it's outside the middle 80%).
Which means you realistically need 700 or greater to be competitive. Otherwise, it's going to be a roll of the dice.
As for taking extension classes to address a low GPA, the standard practice is to take quant courses - college level calculus, algebra, statistics, microeconomics, macroeconomics. No brownie points in this area for doing something different. Just take 1 or 2 of these courses (freshman/intro level courses are sufficient).
As for the spring admissions stats, I don't know offhand.
In any case, your entire focus right now should be on the GMAT. Do whatever it takes - take some courses, hire a tutor, make it your priority. With a mid to high 600s score you won't be shut out, but you will need a bit of luck on your side (assuming your application is well executed, which is a must regardless of your GMAT score). The GMAT isn't everything, but it's a critical first hurdle (sort of like height to basketball).
Follow me on Facebook