FYI - back glass in Gorilla Glass 3 only in ZF6. Prove at Corning website.
Everyone knows this. Gorilla Glass 3 is more scratch resistant than newer generations, which makes sense given you're going to be putting it on hard surfaces.
Everyone probably has possibility to know about Gorilla Glass 6 in Zenfone 6. This info was in tech specs but no one seen words "Gorilla Glass 3" there... ?