I just use water but I've read that if you're gonna give your dog/s Gatorade, use a 50/50 Gatorade/water mix. Or better yet, use a 50/50 mix of Pedialyte/water. Pedialyte has the electrolytes but has less sugar. However, since dogs only sweat through their paws, my guess is they lose electrolytes at a much slower rate than humans.
Good question for a vet though.