The fact is; Apple has a patent of utilizing thin sapphire screens with thin IZGO Indium/zinc/gallium oxide to make a solar cell. In addition, the sapphire top coat in also needed for the finger print ID.
I don't believe Gorilla glass can match this combination. Also, GTAT claims they can get close to matching the cost of gorilla glass.
"The iPhone 6 may be linked to a new innovative and unique development. Several sources, including Business Today’s website, have found significant evidence that Apple is rumored to be working to implement a Sapphire solar power screen for the iPhone 6".