In situations of sever dehydration, salt loss is as important as fluid loss.
Salt is passively absorbed from the gut, whereas sugars are (for the most part) actively absorbed through a transporter.

One of the transporters involved in glucose uptake is a sodium/glucose cotransporter. For every molecule of glucose absorbed, a molecule of sodium is also absorbed. This is the basis of the WHO oral rehydration solution which contains 25 gm of carbohydrates and 3 gm of salt or 1.2 grams of sodium per liter.

Gatorade contains 64 grams of carbohydrate and 450 mg of sodium per liter. So twice as much sugar and half as much salt. Undoubtedly done to make it more palatable. This still takes advantage of the Na/glucose cotransporter. The excess carbohydrate is absorbed through other means.

Gatorade is far from perfect. As a pediatrician, before the days of pedialyte, we would instruct parents to give 1/2 strength gatorade with added salt. Not perfect, but it worked.

As for the sugary solutions not to be given during diarrhea, it is true that unabsorbed sugars cause diarrhea, though in the range we are discussing most are absorbed, as long as the ratio of fructose to glucose is more or less equal (not the case in apple juice btw). But sugary fluids would have no benefit, only potentially downside. But sugary solutions with salt have a significant benefit.