NCC:
I agree with the previous post. The truth is there is no "extra" course that truly compensates for a low(er) GMAT. GMAT and GPA usually go hand in hand. Ideally both would be great, but if one of the two is strong (and the other is not) it CAN help compensate for the weaker of the two.
The only other way to balance out a low GMAT is if you have significant work experience that is in that function. So, for example, if you have a low Quant score, it would help if you were working in finance or another technical field. The schools want to see that you can handle the academic rigor of the MBA program. The GMAT (and the quant section in particular) is one somewhat objective way to measure that.
Unfortunately, all of these extra courses are not going to move the needle. They are viewed as "fluff" and don't carry really any weight. As noted in the other post, you are better off putting the time into a GMAT prep course or focusing on your applications if you are actively applying.
Hope that helps,
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