The reason why Gatorade is recommended over water during dilution techniques is because it helps in the specific gravity department.

Labs look for overdiluted samples by checking both the creatinine and specific gravity for low numbers. Drinking a bunch of water lowers the creatinine and S/G to where the sample can get flagged for being too dilute. Therefore, when utilizing dilution techniques, an effort must be made to keep the creatinine and S/G within the satisfactory ranges. The steps to be taken to accomplish this is all outlined in N2s Dilution "sticky" thread.