If you mean that you want to buy a GPS that comes with the Whitney trail map preloaded on it from in the store, I'm not sure if that exists. Garmin sells an SD card that has topo maps for the west coast that you can just plug in, but it only works on their more expensive units. Otherwise you will have to load the map from your computer before you go.
Your phone should last if you leave it turned off when you aren't using it. (And if it's on, have it in airplane mode unless you actually need signal) I'm planning on taking my GPS to Whitney because I have it and it doesn't weigh much, but if we need a map we'll look at the paper one or the topo map app on someone's phone because it has much better resolution than my low end GPS unit.
