Smart Compose is a feature in Gmail that uses machine learning to figure out what you want to say before you write it. Basically it’s just a word prediction feature, and on a computer it allows you to hit “Tab” to fill in sentences as you’re writing them. It’s a great feature and relatively accurate, though it can be quite a bit off sometimes. Thankfully it’s purely optional and even when enabled is entirely unobtrusive.
The feature also resided on mobile, but it was exclusive to the Pixel 3 and Pixel 3 XL. You would use a swipe gesture to fill in sentences much like the web version of Gmail. However, Google is now rolling this feature out to other devices outside of Google’s own.
I’ve already received the feature on my Huawei Mate 20 Pro and though it’s a bit slow, it works just as well as it does on a computer. The swipe gesture is easy and if you don’t want to use it, it doesn’t get in the way. This is likely a server side roll out so you may need to give it a few days before your devices get this feature. You’ll need to be running Gmail version 9.2.3 and wait for the prompt telling you to give it a try. And if you want to disable it, head to Settings > your account > Smart Compose.