I found the pre-mixed sachets in small zip lock bags keep okay, but get chunky in humid environments. I never wanted to pre-mix a larger batch just in case the granules of different sizes separated out, altering the recipe.
Gatorade is crap. If you look at the sodium and potassium content it's about 2% of your daily value. That's the same as NOTHING. The WHO recipe has over 50% of your daily value of sodium.
V8 juice actually has nearly 10mEq of potassium and PLENTY of sodium, and chased with a glass of water would be better than gatorade. For serious rehydration, you want the WHO formula because V8 will delay gastric emptying, but gatorade is just expensive colored sugar water.
PS I'll take the taste of old socks over a 14gauge needle in my arm any day!