There are three big names in ion exchange glass:
- Gorilla Glass by Corning (US)
- Xensation by Schott AG (Germany)
- Dragontrail by Ashashi (Japan)
I think they all have licensed their glass to factories in multiple countries, so it can be made wherever it's needed.
All of them are used in smartphones and tablets.
My guess is that Apple doesn't specify, in order to be able to switch suppliers on the fly.
Apple posted this a couple of years ago when they were talking about iOS creating US jobs:
"This figure also includes workers in Texas who manufacture processors for iOS products, Corning employees in Kentucky and New York who create the majority of the glass for iPhone, and FedEx and UPS employees."
No mention of GG by name, however.
But there was the whole thing in Jobs' biography about him going to Corning's CEO and convincing him to convert factories to making Gorilla Glass in quantity.
I think the most likely explanation for Apple's lack of specifying GG explicitly is because they use multiple suppliers for iPhone glass. I suspect that even within the same product line, some batches will use GG but others may use Dragontail or another brand of glass.
When iPhone was first introduced in 2007, the production ramp-up was minuscule compared to a new iPhone launch today. Apple would be wise not to rely on a single supplier for any critical component, as they need to build millions and then tens of millions of units very quickly each year.
My guess is that Apple has its own specifications for glass scratch resistance, shatter resistance, clarity, and other attributes. They buy from vendors that can meet these specs. It would explain some of the variation in glass durability that seems to be reported in the iPhone forum here.