We've asked Google if it can say just how large this deployment is, although it's not limited by region.

The functionality is the same, no matter what device you're using. Smart Compose uses a combination of your writing habits and common statements to suggest phrase segments as you type -- you just have to swipe right to use them. If you type "how are," for instance, Gmail will likely suggest "how are you?" as your intended phrase. It risks leading to cookie cutter replies, but it could also save you time when writing perfunctory business emails or confirming a lunch date.