It reminds me of when the FTC challenged an advertisement by Nestle that claimed Carnation Instant Breakfast has as much mineral nourishment as two slices of bacon. The problem? Bacon has no minerals.
The FTC encourages advertisers to run comparative ads - they're viewed as beneficial to consumers so long as they don't mislead. Coke's statement that Gatorade is incomplete without an insignificant amount of calcium and magnesium isn't a comparative ad - it is a claim and ad claims must be substantiated. Coke will have a tough time substantiating a claim that Gatorade is incomplete without an insignificant amount of minerals.
It looks like Coke might just lose the Pepsi challenge again.