I've had two sudden losses of pressure on GP4000s. One was a sidewall blowout - I assume there must have been debris but I never found anything that would have caused the cut. The other was the bead simply lifting off the rim (DA 7900 CLs) for no apparent reason. Both on the rear while riding in a straight line fortunately. I don't like how easy they are to slip on the wheel - easy on has to be easy off, right?
Shame, because they are a great tyre, ride almost as good as Michelin PR3s and have a lot less punctures. But I am starting to lose a bit of confidence in them.