Smart Compose is essentially Smart Replies for Emails. Instead of cold, robotic canned replies, we get full sentences. We got out first look at Smart Compose for Gmail at I/O 2018. The feature then made its way to web versions of Gmail followed by G-Suite users. Smart Compose predicts commonly used phrases and attempts to complete your sentences, based on your typing patterns. It was meant to be a Pixel 3-only exclusive, but it looks like Smart Compose is making its way to all versions of Gmail on Android.
Smart Compose is now live my Galaxy S9+. I haven't been able to see much of it in action just yet, but I'll be sure to include some screencaps when I do. After updating Gmail to the latest version, you'll get a pop-up explaining what the feature is when you start typing an email, and after you've written a few words, you'll get a suggestion. Swipe on the text to accept the suggestion, or continue typing if you want to say something else.
Much like other "AI-Driven" features, Smart Compose gets better over time as it analyses more of your typing patterns and sentences. It is unclear how much of the data goes back to the mothership, and at this point, it's hard to speculate. The feature is auto-enabled by default but can be switched off if needed. You need to be running version 9.2.3 of Gmail. If you're on an older version, you should receive an OTA shortly or can download it via this link.