I've had some on the back of the truck and got caught in a surprise thundershower where it got a pretty good soaking. Let it dry out on the back of the truck in the sun the next day (black, plastic bedliner helps) and hung it with no problem....
When the sheets of drywall are completely dried out, you'll know. If the paper is smooth and the gypsum strong you're good to go. If the paper is puckered and/or the gypsum is soft and swollen, toss it.
Again, I really think you'll know just by looking at it closely.
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