Smart Compose is a useful addition to Gmail. Based on your email writing patterns and some commonly used phrases, it tries to predict what you will say next and surfaces it as a suggestion that you can simply accept. The feature began as an experimental feature on the desktop, then was added to G Suite accounts, but it wasn't until the October Pixel event that we learned it'd be coming to phones as a Pixel exclusive. However, it turns out the feature would actually be exclusive to the Pixel 3, and not even users of the Pixel 2 or any other devices could benefit from it. That's changing today.
Smart Compose is now live on both my Pixel 2XL and OnePlus 6T, and we've also verified that it's working for others on their Pixel 2 and Galaxy S9+ devices. You'll get a pop-up explaining what the feature is when you start typing any email, and after you've written a few words, you'll get a suggestion. Simply swipe on the text (not the keyboard) to accept the suggestion, or continue typing if you want to say something else.
The feature is auto-enabled, but you can turn it off if you prefer by going to your Gmail settings, opening the account where you want it disabled, and ticking off the Smart Compose box. Our devices are running version 9.2.3 of Gmail (APK Mirror), so if you don't see the option yet, make sure you're on that version and close then reopen the app. That should trigger it to show up.