Gatorade and similar products contain all sorts of things I would give my cats - or myself, come to that. Colouring, for example. It's easy to make oral rehydration fluid - there are lots of recipies on the Internet - but I suspect the cat is fully rehydrated by now, in which case giving it oral rehydration is pointless.
The OP can tell by lifting the scruff of it's neck. If it pings straight back down like elastic the cat is fine.