I've backpacked the GC several times but never in summer.....for a reason.

But I have done a lot of hiking/backpacking in the Mojave Desert in hot weather. You've heard a lot of good advice here. All I would add is choose a very light color, preferably white. When the sun is that intense darker colors really suck in the heat. My ideal very hot weather garment is a 100% white cotton LS T shirt which I try to keep soaked if practical.
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If you only travel on sunny days you will never reach your destination.*

* May not apply at certain latitudes in Canada and elsewhere.